Prepared in cooperation with the Idaho Department of Water Resources

Recharge Sources and Residence Times of Groundwater as 

Determined by Geochemical Tracers in the Mayfield Area, 

Southwestern Idaho, 2011–12

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2013–5115

Recharge Sources and Residence 

Times of Groundwater as Determined by 

Geochemical Tracers in the Mayfield Area, 

Southwestern Idaho, 2011–12

By Candice B. Hopkins

Prepared in cooperation with the Idaho Department of Water Resources

Scientific Investigations Report 2013–5115

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director 

U.S. Geological Survey, Reston, Virginia: 2013

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Suggested citation:

Hopkins, C.B., 2013, Recharge sources and residence times of groundwater as determined by geochemical tracers in 

the Mayfield Area, southwestern Idaho, 2011–12: U.S. Geological Survey Scientific Investigations Report 2013–5115, 

36 p., http://pubs.usgs.gov/sir/2013/5115/.
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Purpose and Scope ..............................................................................................................................3

Previous Investigations........................................................................................................................3

Description of the Study Area .....................................................................................................................4

Land Use.................................................................................................................................................4

Climate....................................................................................................................................................4

Geology...................................................................................................................................................5

Hydrogeology.........................................................................................................................................5

Methods...........................................................................................................................................................7

Field Methods........................................................................................................................................7

Site Selection ...............................................................................................................................7

Sample Collection........................................................................................................................8

Quality Assurance/Quality Control............................................................................................8

Laboratory Methods.............................................................................................................................8

Flow-Path Cross Plots..........................................................................................................................9

Carbon-14 Age Interpretation and Correction .................................................................................9

CFC Calibration Using Dissolved Gas Data.....................................................................................10

Geochemical Modeling......................................................................................................................10

Cross Section Construction ..............................................................................................................11

Concentration and Distribution of Geochemical Tracers......................................................................11

Geochemical Evidence for Recharge from Upwelling Geothermal Waters ............................30

Sources of Recharge as Suggested by Geochemical Tracers ...................................................32

Summary and Conclusions.........................................................................................................................33

Acknowledgments ......................................................................................................................................34

References Cited..........................................................................................................................................34
iv

Figures

1. Map showing Mayfield study area and water-quality sampling locations,

southwestern Idaho .....................................................................................................................2

2. Graph showing discharge at U.S. Geological Survey streamflow-gaging station

Indian Creek near Mayfield, Idaho, July 2010–July 2012 .......................................................6

3. Graphs showing major ion concentrations along a relative flow path in water

samples collected in wells from the Mayfield area, southwestern Idaho, 2011–12........13

4. Graph showing nitrate concentration compared with carbon-14 in water

samples collected from the Mayfield area, southwestern Idaho, 2011–12 ......................15

5. Graph showing concentrations of arsenic and lithium along a relative flow path

in water samples collected from wells from the Mayfield area, southwestern

Idaho, 2011–12.............................................................................................................................19

6. Graph showing oxygen and hydrogen isotopes from groundwater and surface

water in the Mayfield area, southwestern Idaho, 2011–12, plotted with a global

meteoric water line (GMWL) and two local meteoric water lines (LMWLs)....................21

7. Cross section showing selected wells and percentages of young water in

samples collected in the Mayfield area, southwestern Idaho, 2011–12 ...........................25

8. Graph showing pH and fluoride concentrations in water samples collected from

wells in Mayfield area, southwestern Idaho, 2011–12 .........................................................31

9. Ternary diagram showing water samples plotted with Idaho Batholith thermal

water, Mayfield area, southwestern Idaho, 2011–12............................................................31

Tables

1. Summary of data from selected weather stations near the Mayfield area,

southwestern Idaho, 2011–12 .....................................................................................................4

2. Site location information for wells and surface-water sites sampled in the

Mayfield area, southwestern Idaho, 2011–12 ..........................................................................7

3. Field parameter values for water samples collected in the Mayfield area,

southwestern Idaho, 2011–12 ...................................................................................................11

4. Major-ion concentrations in water samples collected from the Mayfield area,

southwestern Idaho, 2011–12 ...................................................................................................12

5. Nutrient concentrations in water samples collected in the Mayfield area,

southwestern Idaho, 2011–12 ...................................................................................................14

6. Metals and trace-element concentrations for water samples collected in the

Mayfield area, southwestern Idaho, 2011–12 ........................................................................16

7. Isotope data and corrected carbon-14 ages for water samples collected in the

Mayfield area, southwestern Idaho, 2011–12 .......................................................................20

8. Dissolved gas data and estimated recharge temperatures for water samples

collected in the Mayfield area, southwestern Idaho, 2011–12 ...........................................23

9. Chlorofluorocarbon results and simulated groundwater recharge dates based

on recharge elevation for groundwater samples collected in the Mayfield area,

southwestern Idaho, 2011–12 ...................................................................................................24

10. Saturation indices of selected water samples from wells in the Mayfield area,

southwestern Idaho, 2011–12 ..................................................................................................26

11. Geochemical model simulations results using NETPATH, measured carbon-14

and calculated carbon-14 ages, and flow path modeling travel times for

groundwater in the Mayfield area, southwestern Idaho, 2011–12.....................................27

12. Validity of mixing models using NETPATH, Mayfield area, southwestern Idaho,

2011–12 .........................................................................................................................................28
v

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

 Length

inch (in.) 2.54 centimeter (cm)

foot (ft.) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Flow rate

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

Hydraulic gradient

foot per mile (ft/mi) 0.1894 meter per kilometer (m/km)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: 

°C=(°F-32)/1.8

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25 °C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).

Datums

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the vertical datum.

Abbreviations and Acronyms

CFC chlorofluorocarbon

GMWL Global Meteoric Water Line

IDWR Idaho Department of Water Resources

LMWL Local Meteoric Water Line

NWISWeb USGS National Water Information System website (NWISWeb)

pmc percent modern carbon

RASA Regional Aquifer System Analysis

SRP Snake River Plain 

USGS U.S. Geological Survey

VSMOW Vienna Standard Mean Ocean Water
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Recharge Sources and Residence Times of 

Groundwater as Determined by Geochemical Tracers in 

the Mayfield Area, Southwestern Idaho, 2011–12

By Candice B. Hopkins

Abstract

Parties proposing residential development in the area of 

Mayfield, Idaho are seeking a sustainable groundwater supply. 

During 2011–12, the U.S. Geological Survey, in cooperation 

with the Idaho Department of Water Resources, used 

geochemical tracers in the Mayfield area to evaluate sources 

of aquifer recharge and differences in groundwater residence 

time. Fourteen groundwater wells and one surface-water site 

were sampled for major ion chemistry, metals, stable isotopes, 

and age tracers; data collected from this study were used to 

evaluate the sources of groundwater recharge and groundwater 

residence times in the area. 

Major ion chemistry varied along a flow path between 

deeper wells, suggesting an upgradient source of dilute water, 

and a downgradient source of more concentrated water with 

the geochemical signature of the Idaho Batholith. Samples 

from shallow wells had elevated nutrient concentrations, a 

more positive oxygen-18 signature, and younger carbon-14 

dates than deep wells, suggesting that recharge comes from 

young precipitation and surface-water infiltration. Samples 

from deep wells generally had higher concentrations of 

metals typical of geothermal waters, a more negative 

oxygen-18 signature, and older carbon-14 values than 

samples from shallow wells, suggesting that recharge comes 

from both infiltration of meteoric water and another source. 

The chemistry of groundwater sampled from deep wells is 

somewhat similar to the chemistry in geothermal waters, 

suggesting that geothermal water may be a source of recharge 

to this aquifer. Results of NETPATH mixing models suggest 

that geothermal water composes 1–23 percent of water in deep 

wells. Chlorofluorocarbons were detected in every sample, 

which indicates that all groundwater samples contain at least a 

component of young recharge, and that groundwater is derived 

from multiple recharge sources. Conclusions from this study 

can be used to further refine conceptual hydrological models 

of the area.

Introduction

The population of Ada County, Idaho, has increased 

from about 112,000 in 1970 to 373,406 in 2007 (Church, 

2007). The Community Planning Association of Southwestern 

Idaho predicts that the population of Ada County will grow to 

875,960 by 2040 (Church, 2007). Development (8,000 homes) 

is planned for eastern Ada County and western Elmore County 

near the Mayfield town site (fig. 1). A sustainable water supply 

is crucial to the future development of the Mayfield area. That 

supply could come entirely from Mayfield area groundwater, 

or from groundwater used in combination with water brought 

in by pipeline from an outside area. To ensure a sustainable 

water supply, the Idaho Department of Water Resources 

(IDWR) is charged with the issuance of water rights, and is 

conducting studies to determine long-term water supply and 

possible sources of interference with existing water rights.

Limited information is available on the hydrogeology 

of the area. Recent efforts by IDWR have included waterlevel monitoring and interpretation. Studies commissioned by 

developers have assessed potential groundwater availability in 

the area, but additional questions exist about the sustainability 

of groundwater. Some of these questions relate to the amount 

and sources of recharge. A better understanding of recharge 

can help to constrain conceptual models of the hydrology, and 

to guide future decision making. This study was conducted 

by the U.S. Geological Survey (USGS) in cooperation 

with IDWR. It was initiated to determine the sources of 

groundwater recharge and groundwater residence times in the 

Mayfield area by examining geochemical characteristics of 

groundwater occurring along a flow path, and differences in 

the geochemistry of groundwater with depth. 
2 Recharge Sources and Residence Times of Groundwater in the Mayfield Area, Southwestern Idaho, 2011–12

tac13-0813_fig01

ADA

ELMORE

30

84

PICR1

PICR1

D4

D3

P1

P1

D2

P5

P3

D1

M1

M1

P2

D6

D6

D5

D7

P4 SW1

SW1

Indian

Creek

Blacks Creek

Bryans Run

Soles Rest Creek

Browns Creek

Ditto Creek

Dry Creek

Willow 

 Creek

Squaw Creek

Mud Springs Creek

Creek

North Indian Creek

Beaver Creek

Cow Creek

Slater

Slater Creek

Slater Creek

West Fork 

Browns Creek

116°W 115°50'W

43°30'N

43°20'N

0 1 2 3 4 MILES

0 1 2 3 4 KILOMETERS

Cross section

Sample locations and identifier

Perched aquifer near 

Indian Creek Reservoir

Mountain system

Perched aquifer

Deep aquifer

Surface water

EXPLANATION

A A’

SLATERS FLAT

S A N D H O L L O W

DANSKIN MOUNTAINS Lydle Gulch

Caldwell

Draw

Mayfield

M c L n t r y r e D r a w

Smith Draw

Orchard

A

A’

Indian Creek 

Reservoir

Study Area

IDAHO

Boise City

Mountain Home City

Base map from The National Map and the National Hydrography Dataset.

Projection: North America Albers Equal Area, NAD83.

Figure 1. Mayfield study area and water-quality sampling locations, southwestern Idaho.
Introduction 3

Purpose and Scope

This report identifies sources of recharge and the 

residence times of groundwater in the Mayfield area of 

southwestern Idaho (fig. 1) by characterizing isotopic and 

environmental tracers in groundwater and surface water. It 

includes brief discussions of environmental tracers, how they 

are used to better understand hydrogeology, and the waterquality analyses for samples collected from 14 wells and 

1 stream during 2011–12. 

Previous Investigations

Several regional studies have examined the groundwater 

geochemistry of southwestern Idaho, including the Mayfield 

area. Although most of these studies included little data from 

the Mayfield area itself, they did provide valuable background 

information about the water quality of the area.

Wood and Low (1988) presented work done as part 

of the USGS Snake River Plain (SRP) Regional Aquifer 

System Analysis (RASA) program. Because the RASA was 

a regional study, few samples were collected and analyzed 

in the Mayfield area. The main focus of this work was to 

identify the chemical reactions between the aquifer matrix 

and groundwater that control solute concentrations in the SRP 

aquifer system. 

Parliman and Young (1992) described selected 

well‐inventory and water-chemistry data for 718 thermal‐water 

wells and springs in Idaho, including four near the Mayfield 

area. Analyses included major ions, nutrients, bacteria, trace 

elements, and oxygen/deuterium. 

Neely and Crockett (1998) presented water-quality 

data collected from 281 wells in the Treasure Valley area 

(144 wells were less than 250 ft depth; 137 wells were 

greater than 250 ft depth), including a number of wells near 

the Mayfield area. The data collected included major ions, 

nutrients, and trace elements. The study included an evaluation 

of network design, trend analysis of selected constituents, and 

scatter plots and trilinear diagrams. 

Hutchings and Petrich (2002) sampled 38 wells in 

the Boise River drainage basin. Only one of the wells 

sampled for that study is near the Mayfield area. Analytes 

included major ions and carbon isotopes. Radiocarbon was 

analyzed in a subset of 28 wells; a carbon-14 correction 

model based on carbon-13 values was recommended. The 

youngest waters were located adjacent to the northern 

foothills, increasing in age towards the western edge of the 

basin near the Snake River with a corresponding increase in 

groundwater mineralization. 

Reports focusing specifically on the Mayfield study 

area have been published as interest in the development 

of the study area has increased. SPF Water Engineering 

(2007a) conducted a preliminary study to determine the 

availability and sustainability of groundwater in the vicinity 

of Mayfield. The study also examined water demand, local 

geology, drillers’ reports, and the possible effects of pumping. 

Suggested recharge sources were seepage from surface 

channels, infiltration of precipitation, and underflow from 

the Danskin Mountains. SPF Water Engineering did not 

suspect discharge from the shallow aquifers into Indian Creek. 

Geochemical data collected for the SPF Water Engineering 

(2007a) study included major ions and nutrients; estimates 

for evapotranspiration, stream seepage, and precipitation also 

were included in this study. 

In a follow up study, SPF Water Engineering (2007b) 

examined rising groundwater levels in a proposed 

development to the south of the Mayfield area. The study 

examined previously available water-quality data from 

the Idaho Statewide Ground Water Quality Monitoring 

Program; the authors suggested the area had “good” water 

quality. Water temperature and arsenic concentrations were 

noted to increase with depth. The SPF Water Engineering 

(2007b) report also contains information on precipitation and 

evaporation, infiltration estimates, and a water budget. In 

addition, geological information available from drillers’ logs 

was described. 

Welhan (2012) evaluated previously existing geochemical 

and hydrogeological data in the Mayfield study area. Possible 

recharge was determined to be from infiltration of surface 

water, precipitation or meteoric recharge from local drainage 

basins, and upwelling of deep geothermal water. Recharge 

estimates based on pumping tests suggested that previous 

estimates of recharge were too high. The Welhan (2012) study 

recommended that additional geochemical and groundwater 

temperature data be collected from the Mayfield study area. 

IDWR has published numerous technical memos that 

describe the hydrology and geology of the Mayfield area. In 

addition, several groundwater studies have been conducted 

near the study area due to its proximity to areas of increased 

concern over groundwater supply. The Mayfield study area 

overlaps the Mountain Home Ground Water Management 

Area. Additionally, several proposed developments in the 

study area are within 5 mi of the northern boundary of the 

Cinder Cone Butte Critical Groundwater Area. IDWR has 

designated these areas as areas of groundwater concern 

because they seem to have, or may be approaching, an 

insufficient groundwater supply. Studies conducted on 

these specific groundwater areas (Norton and others, 1982; 

Bendixsen, 1994) offer insight into regional groundwater 

movement and trends. 
4 Recharge Sources and Residence Times of Groundwater in the Mayfield Area, Southwestern Idaho, 2011–12

Description of the Study Area

The Mayfield study area (fig. 1) spans parts of Ada and 

Elmore Counties between Boise and Mountain Home, Idaho. 

A large portion of the study area is slated for development, 

including an area that lies north of U.S. Interstate Highway 

84 and south of the Danskin Mountains in the upper Indian 

Creek drainage basin. Land surface altitude within the 

study area ranges from 6,700 ft in the Danskin Mountains 

to 3,140 ft near the historic town of Orchard in the southern 

part of the study area.

Land Use

The study area is sparsely populated, but it is being 

considered for potential development. As of October 2008, 

11 water rights applications were pending for planned 

communities in or near the study area (Idaho Department of 

Water Resources, written commun., 2011). The ecoregion of 

the area is defined as a mixture of Mountain Home Uplands 

and Foothills Shrublands-Grasslands (National Agricultural 

Statistics Service, 2011). Non-developed land is covered by 

shrub land or pasture area. Additionally, the area has a small 

amount of agricultural land use for hay, alfalfa, corn, winter 

wheat, and sugar beets. The foothills and benches are dry, 

generally treeless, and covered by shrubs and grasses. Most 

of the land is used for grazing, but a small part of the land 

has been developed for residential use (U.S. Department of 

Agriculture, 2012).

Climate

Climate in the Mayfield study area falls into two 

classifications under the modified Köppen system: the plains 

below the foothills are classified as BSk, and the foothills are 

classified as Dsa (Godfrey, 2000; Peel and others, 2007). For 

the plains, BS indicates an arid steppe climate, and k indicates 

that the mean annual air temperature is less than 17.8°C. 

For the foothills, Dsa identifies a cold climate with dry, hot 

summers (Peel and others, 2007).

Four weather stations surrounding the study area have 

sufficient long-term data for climatic calculations (table 1). 

As of 2012, one active Agrimet station is within 25 mi of 

the study area: Boise, Idaho (Bureau of Reclamation, 2006). 

Additionally, four Natural Resources Conservation Service 

snow-survey sites are in or adjacent to the study area: three are 

instrumented SNOTEL sites, and the other is a snow course 

site (Natural Resources Conservation Service, 2010). 

Long-term mean air temperatures for selected National 

Weather Service stations are listed in table 1. Mean annual 

air temperatures at the four stations range from 10.7°C at 

Boise Weather Service Field Office (WSFO) Airport to 

11.5°C at Boise Lucky Peak Dam. The coldest month in the 

area is January, with mean low air temperatures ranging from 

-6.3°C at Mountain Home and Grand View 2W to -5.4°C at 

Boise WSFO Airport. Typically, the warmest month is July, 

with mean high air temperatures ranging from 32.5°C at 

Boise WSFO Airport to 34.6°C at Grand View 2 W. Mean 

annual precipitation ranges from 7.15 in. at Grand View 

2 W to 13.7 in. at Boise Lucky Peak Dam (table 1). July and 

August typically are the driest months; November, December, 

and January are the wettest (Western Regional Climate 

Center, 2008).

Table 1. Summary of data from selected weather stations near the Mayfield area, southwestern Idaho, 2011–12.

[Climate data from Western Regional Climate Center (2008). Abbreviations: °C, degrees Celsius; in., inch; ft, foot; NAVD 88, North American Vertical Datum 

of 1988; WSFO, Weather Service Field Office]

Station name

and No. Latitude Longitude

Mean air temperature

(°C) Mean total 

precipitation

(in.)

Mean total 

snowfall

(in.)

Altitude 

(ft above 

NAVD 88)

Dates in operation

July 

maximum

January 

minimum Annual Startingdate

Ending

date

Mountain Home,

Idaho, 106174

43°08'N 115°42'W 34.0 -6.3 10.8 10.6 11 3,190 08-01-1948 10-31-2007

Grand View 2 W,

Idaho, 103760

43°00'N 116°08'W 34.6 -6.3 11.2 7.15 5.4 2,400 04-01-1933 12-31-2005

Boise Lucky Peak 

Dam, Idaho, 101018

43°33'N 116°04'W 33.1 -6.2 11.5 13.7 4.9 2,840 01-12-1951 12-31-2005

Boise WSFO

Airport, Idaho, 

101022

43°34'N 116°13'W 32.5 -5.4 10.7 11.8 20 2,840 01-01-1940 12-31-2005
Description of the Study Area 5

Geology

The Mayfield study area is on the edge of the western 

SRP, located between the northern Rocky Mountain 

physiographic province and the northern Basin and Range 

Extensional Province (Wood, 1994). The western SRP is a 

northwestern-trending intracontinental rift basin, 28-mi wide 

and 155-mi long. It was formed by volcanism associated with 

the passage of the Yellowstone plume system during the late 

Cenozoic Period, and by Basin and Range extension during 

the Pleistocene (Shervais and others, 2002). 

The general geology of the western SRP consists of 

Cretaceous biotite-granodiorite Idaho Batholith basement 

rock, overlain by 1.6–2.7 mi of Miocene basalt, and covered 

by lacustrine and fluvial sediments (Mitchell, 1981; Wood 

and Anderson, 1981; Mayo and others, 1984; Whitehead, 

1986; Wood and Clemens, 2002). The bulk of sediments in 

the study area were deposited during the Pliocene by Lake 

Idaho, a long-lived system that once filled the western SRP to 

an altitude of 3,600 ft (Shervais and others, 2002). Because 

the study area is located on the edge of the basin, mostly 

fine-grained, near-shore Lake Idaho lacustrine sediments 

are present (Welhan, 2012). Atop the sand and Lake Idaho 

sediments sit sand and pebbles derived from the granitic 

highlands (Phillips and others, 2012).

Idaho group sands in the Mayfield area are interfingered 

with Pleistocene basalt flows and dominate the subsurface, 

increasing in thickness to the south (Liberty, 2012). The 

Mayfield study area has outcrops of Eocene rhyolitic tuff 

and lava, as well as outcrops of Miocene basaltic tuff and 

tuffaceous sandstone near the Danskin Mountains. Basaltic 

units in the local area include the Slaters Flat shield Volcano, 

as well as Pleistocene and older basalt flows that are common 

in the western SRP (Phillips and others, 2012; Welhan, 2012). 

Sands and gravels contain quartz, orthoclase, plagioclase, 

biotite, and mafic minerals; some grains are coated with 

carbonate-silica to form oolites (Phillips and others, 2012).

Hydrogeology

The main water-bearing units in the Mayfield area are 

Idaho group sands and underlying fractured basalts. Welhan 

(2012) used drillers’ reports to infer the locations of some of 

the principal water-bearing zones in the study area. Water is in 

shallow, perched zones (hereafter called the perched aquifer) 

that may or may not be laterally continuous; water is also in a 

more continuous, unconfined deep aquifer below the perched 

water table (hereafter called the deep aquifer) (Welhan, 2012). 

At this date (2013), confining units have not been identified, 

although Welhan (2012) suggests that cementation of sand 

and silica could serve as a local confining unit if laterally 

continuous. Additionally, clay layers from near-shore lake 

deposits could form discontinuous impermeable layers. 

Multiple studies have concluded that groundwater 

generally flows southwestward from the study area towards 

the Snake River (SPF Engineering, 2007a; Welhan, 2012). 

A hydraulic gradient of about 133 ft/mi towards the 

southwest was determined from cross sections, but did not 

differentiate between shallow and deep water-bearing zones 

(S.H. Wood, Boise State University, unpub. report, 1996). 

Hydraulic gradients in the deep aquifer are steepest about 

4–5 mi southwest from the range front (Welhan, 2012). The 

steepening of the hydraulic gradient away from the range front 

suggests that geologic controls (buried faults) or local recharge 

may play a role in shaping the local water table.

It has been suggested that faulting controls groundwater 

flow in the area (Liberty, 2012; Welhan, 2012). The 

northeast‐trending Blacks Creek fault offset Miocene dikes 

(Phillips and others, 2012), which may suggest similar 

faulting. Liberty (2012) suggested that buried faults exist near 

the range front. Northeast-trending, high-angle faults and 

shear zones in the Idaho Batholith exist in the Batholith to the 

north of the study area and may influence the groundwater 

flow in the study area (O’Neil and Lopez, 1985) by facilitating 

flow between surface water, the perched aquifer, and the deep 

aquifer. Welhan (2012) speculates that contact between granite 

and sediment near the mountain front may be cut by two small 

displacement faults; these faults could possibly provide a 

mechanism for surface water or perched groundwater to move 

into the deep aquifer. Faults also could impede groundwater 

flow, forming a barrier between aquifer units, or possibly 

breaking up the perched aquifer into discontinuous perched 

water bodies. 

Several possible sources of recharge to the Mayfield 

study area have been suggested. These include infiltration of 

precipitation in excess of evapotranspiration, seepage losses 

from ephemeral streams that flow onto alluvial fans, mountain 

front recharge, and upwelling of water from interbasin flow 

or deep geothermal waters (SPF Water Engineering, 2007a; 

Welhan, 2012). SPF Water Engineering (2007a) estimated 

recharge and evapotranspiration rates in the area and 

determined that only a small amount of precipitation actually 

infiltrates and contributes to recharge in the study area. No 

studies to date have quantified seepage from surface water 

into alluvial fans or mountain system underflow. Williams 

and Etheridge (2013) determined that there is little to no 

seepage from Indian Creek Reservoir (fig. 1) to the underlying 

deep aquifer. Underflow from the Idaho Batholith could also 

be contributing recharge to the study area; Newton (1991) 

considered this possibility and suggested that the amount 

of recharge from this source would be small due to the 

impermeable nature of the granitic basement rock.
6 Recharge Sources and Residence Times of Groundwater in the Mayfield Area, Southwestern Idaho, 2011–12

Welhan (2012) theorized that upwelling geothermal 

water was a potential source of recharge in the area; he used 

water temperatures to determine that deep wells contained 

as much as 20 percent geothermal water (water warmer than 

16°C). Welhan (2012) proposed the presence of a geothermal 

component in the deep aquifer from groundwater circulated 

at less than 1-km depth. He further postulated that seasonal 

factors, as well as pumping-induced hydraulic stresses, 

may be affecting mixing proportions. The geochemical 

signature of high concentrations of lithium and fluoride that 

characterize the Boise Geothermal system (Waag and Wood, 

1987; Druschel and Rosenberg, 2000) also was detected in 

groundwaters of the study area, further suggesting geothermal 

recharge (Welhan, 2012). 

Surface-water conditions vary throughout the year and 

generally reflect climactic conditions. The dry climate of the 

study area controls streamflow and the ephemeral streams 

in the area generally only flow in response to precipitation 

or snow melt. Discharge, as recorded for 2 years at the 

surface‐water site on Indian Creek (SW1), is an excellent 

example of annual streamflow patterns (fig. 2). Streamflow 

usually peaks around precipitation and runoff events during 

the winter and spring months; typically surface‐water 

flow ceases during the summer and autumn months. 

Similar patterns have been observed in Blacks Creek and 

Browns Creek.

July October January April July October January April

2010 2011

Date

2012

July

Daily mean discharge

EXPLANATION

Daily discharge, in cubic feet per second

USGS 13211100 Indian Creek near Mayfield, Idaho 150

100

50

0

-50

Sample collected 

on January 27, 2012

Figure 2. Discharge at U.S. Geological Survey streamflow-gaging station Indian Creek near Mayfield, 

Idaho, July 2010–July 2012.
Methods 7

Methods

Data for this study were collected from wells selected 

by the USGS and IDWR based on location, well depth, 

well construction, and accessibility. Fifteen samples were 

collected for this study between August of 2011 and March 

of 2012 (fig. 1). Fourteen of the samples were from domestic, 

irrigation, or monitoring wells in the Mayfield area; one 

sample from Indian Creek was collected to characterize 

the water-quality characteristics of surface-water recharge 

(table 2). In addition to the environmental samples, two 

replicate samples were collected for quality-control purposes.

Field Methods

Site Selection

To characterize potential geochemical reactions along a 

flow path, where possible, most wells were selected along a 

line following the hydraulic gradient, or from near the base of 

the foothills southwest toward the SRP. Wells were classified 

by aquifer based on the definition by Welhan (2012) and by 

well completion information.

Table 2. Site location information for wells and surface-water sites sampled in the Mayfield area, southwestern Idaho, 2011–12.

[Depth of well and screening information obtained from well logs associated with station name. Sample names assigned based on depth and aquifer sampled. 

Abbreviations: ft, foot; NAVD 88, North American Vertical Datum of 1988; USGS, U.S. Geological Survey; N/A, not applicable; –, no data available]

USGS station No. Station name

Unique 

station 

identifier

Latitude Longitude

Altitude 

(ft above 

NAVD 88)

Sample

date

Sample 

time

Depth

of well 

(ft below 

land surface)

Depth to water, 

at time of 

sampling 

(ft below 

land surface)

Top of 

screen 

(ft below 

land surface)

Perched aquifer near Indian Creek Reservoir

432310116001001 01N 04E 29DCC1 PICR1 43°23'10.0" 116° 00' 09.8" 3,359 08-04-2011 1538 92 N/A –

Mountain system

432655115543001 01N 05E 06CAB1 M1 43 26' 54.6" 115 54' 29.9" 3,761 08-03-2011 1047 170 17.3 120

Perched aquifer

432343115573701 01N 04E 27ACD2 P1 43°23'42.8" 115°57'37.0" 3,429 11-16-2011 1440 75 N/A 28

432602115554101 01N 04E 12CAC2 P2 43°26'02.3" 115°55'41.4" 3,582 08-02-2011 0950 95 44.2 90

432507115550701 01N 04E 13DAC1 P3 43°25'06.7" 115°55'07.3" 3,571 08-04-2011 1155 100 60.7 95

432450115562401 01N 04E 23ABA1 P4 43°24'50" 115°56'24" 3,517 08-01-2011 1014 147 70.5 –

432501115560701 01N 04E 4DDAD1 P5 43°25'00.8" 115°56'10.3" 3,530 08-02-2011 1140 200 93.8 –

Deep aquifer

432514115555201 01N 04E 13CBA1 D1 43°25'14.5" 115°55'52.3" 3,567 08-02-2011 1335 330 260 320

432430115562001 01N 04E 23ADC1 D2 43°24'29.6" 115°56'20.0" 3,483 11-18-2011 1350 450 N/A 420

432339115573801 01N 04E 27ACD1 D3 43°23'39.2" 115°57'38.7" 3,429 08-04-2011 0955 480 N/A 420

432306115595201 01N 04E 32AAB1 D4 43°23'02.5" 115°59'55.8" 3,370 08-04-2011 1350 711 N/A –

432805116050501 02N 03E 34ACC1 D5 43°28'05.0" 116°05'04.6" 3,417 03-29-2012 0945 861 710 780

431832116010401 01S 04E 30ADC1 D6 43°18'31.8" 116°01'06.5" 3,149 11-22-2011 1317 960 N/A 870

432555116033001 01N 03E 11DDB1 D7 43°25'51.0" 116°03'28.8" 3,355 11-17-2011 0945 1,000 N/A –

Surface water

13211100 Indian Creek near 

Mayfield, Idaho

SW1 43°25'02" 115°53'51" 3,620 01-27-2012 0930 N/A N/A N/A
8 Recharge Sources and Residence Times of Groundwater in the Mayfield Area, Southwestern Idaho, 2011–12

One sample was collected from the mountain system 

(decomposed granite), five samples were collected from the 

perched aquifer (generally shallower than 200 ft depth), and 

seven samples were collected from the deep aquifer (generally 

deeper than 300 ft depth). One sample was collected from 

a locally-perched aquifer near Indian Creek Reservoir, and 

one surface-water sample was collected from Indian Creek. 

Unique site identifiers were assigned to each sample location 

based on which aquifer the water was collected from and can 

be viewed next to the well’s station name in tables 2–9. 

Sample Collection

For groundwater samples, a closed path was established 

between the well and sampling equipment to prevent 

contact with the atmosphere. Water from wells equipped 

with an existing submersible pump was sampled as close 

to the wellhead as possible using the existing plumbing. 

One monitoring well did not have a dedicated submersible 

pump, necessitating the use of a portable electric Grundfos 

Redi‐Flo2® submersible pump and tygon tubing. 

Field parameters (specific conductance, pH, water 

temperature, and dissolved oxygen) were measured at each 

well site using portable meters. Meters were placed in a 

flow-through chamber and connected to the discharge line 

from the well. Samples were collected after at least three 

casing volumes of water were purged from the well and field 

parameters were stable.

Samples were collected in appropriate containers and 

preserved in accordance with the procedures specified in 

the USGS National Field Manual (U.S. Geological Survey, 

variously dated). Bottles for oxygen-18 and hydrogen-2 

analysis were filled with no head space. Bottles for physical 

parameter analyses were rinsed three times, and samples 

were collected without filtration. Bottles for nutrient, metal, 

major ion, and carbon-13 and carbon-14 analyses were 

rinsed three times and filled using water filtered through a 

0.45-micron capsule filter. Carbon-14 samples were collected 

in a safety-coated 1-L glass bottle and shipped chilled. 

Chlorofluorocarbon (CFC) and dissolved gas samples were 

collected using copper tubing. Field procedures followed 

those outlined by the USGS CFC and dissolved gas guidelines 

(U.S. Geological Survey, 2006, 2009). CFC samples were 

collected in glass bottles that were capped with aluminum 

foil-lined caps, sealed with electrical tape, and stored upside 

down. Dissolved gas samples were collected in glass bottles 

and plugged; gasses were allowed to escape during sampling 

through a hypodermic needle injected through the plug.

The surface-water sample was a grab sample collected 

from the centroid of the stream near the streamgage. Field 

parameters were measured in-situ using portable meters. The 

surface-water sample was collected and preserved according 

to procedures specified in the USGS National Field Manual 

(U.S. Geological Survey, variously dated); surface-water 

sample collection did not involve the collection of dissolved 

gases or CFCs. 

Quality Assurance/Quality Control

This study followed the quality-assurance plan for the 

collection of water-quality samples by the USGS Idaho 

Water Science Center (Mark Hardy, written commun., 2008). 

Meters were calibrated in the field each day before use. All 

field equipment was cleaned using established protocols prior 

to use. 

Replicate groundwater samples were collected from two 

wells to identify and quantify the variability (i.e., random 

error) of all aspects of the sampling and analysis process. 

Replicates (also called replicate pairs) consisted of two 

water samples, an environmental sample and a replicate 

sample, with the replicate sample collected immediately after 

collection of the environmental sample. The replicate samples 

were subjected to identical field processing and laboratory 

analysis procedures.

At both sites, the analyte concentrations in the 

environmental and replicate samples were evaluated using the 

relative percent difference equation:

 [( ) / (( ) / 2)] 100

where

is the relative percent difference,

is the value of the primary sample, and

is the value of the replicate sample.

SR SR

S

R

RPD V V V V

RPD

V

V

=− + × (1)

For the replicate samples collected during this study, the 

RPD for all analytes was below 10 percent, indicating good 

reproducibility in sample collection techniques and laboratory 

analysis. Some analyte concentrations were below the 

reporting limit and a RPD could not be calculated; however, 

if both the original and replicate sample plotted near or below 

the reporting limit, the results were considered acceptable.

Laboratory Methods

Samples for major ion, nutrients, metals, and physical 

parameters were analyzed at the USGS National Water 

Quality Lab (NWQL) in Denver, Colo. Laboratory procedures 

are described by Fishman and Friedman (1989) and 

Fishman (1993).
Methods 9

Stable isotopes of oxygen and hydrogen were analyzed 

at the USGS Reston Stable Isotope Laboratory in Reston, Va. 

Samples were collected in glass bottles with polyseal caps. 

The hydrogen equilibration technique (Coplen and others, 

1991) was used to analyze the hydrogen-isotope ratio. Results 

for stable isotopes are expressed in units of per mil (‰, parts 

per thousand) and expressed in delta notation (δ) as derived 

from comparing the sample to Vienna Standard Mean Ocean 

Water (VSMOW) (Coplen, 1996). The two standard deviation 

accuracy of oxygen and hydrogen isotopes were 0.2 and 2‰, 

respectively (Révész and Coplen, 2008a, 2008b).

Carbon-13 (13C) and carbon-14 (14C) isotope samples 

were analyzed at the National Ocean Sciences Accelerator 

Mass Spectrometry Facility in Woods Hole, Mass. Only 

groundwater samples were analyzed for carbon isotopes. 

Carbon dioxide gas was extracted from each sample by acid 

hydrolysis and reduced to graphite for accelerator mass 

spectrometry. Using this technique, samples of carbon up to 

50,000 years old with an error of less than 0.4 percent modern 

carbon (pmc) can be analyzed (Woods Hole Oceanographic 

Institution, 2003). Carbon-13 results are reported in per mil 

(‰, parts per thousand), and expressed in delta notation 

(δ) as compared to a standard reference sample of Vienna 

Pee Dee Belmanite (VPDB) (Woods Hole Oceanographic 

Institution, 2003).

CFCs and dissolved gases were analyzed at the USGS 

Reston Chlorofluorocarbon Laboratory in Reston, Va. CFC 

results were determined in the laboratory using a purge-andtap gas chromatography procedure with an electron capture 

detector (U.S. Geological Survey, 2012a). Dissolved gas 

samples were allowed to equilibrate with headspace with 

the resultant gas analyzed on a gas chromatograph (U.S. 

Geological Survey, 2012b). 

Flow-Path Cross Plots

Cross plots were constructed to show changes in 

groundwater geochemistry along a possible flow path. A 

relative position for a well along a flow path was determined 

based on its position in relation to other wells in the same 

aquifer. Wells are grouped by aquifer and may not represent 

the actual flow path of water. Sites D5 and D7 are plotted 

along the deep aquifer flow path, but may be influenced by 

different water altogether than the other deep wells (D1-D4) 

since they are not in the upper Indian Creek watershed (fig. 1). 

Samples are plotted this way to demonstrate chemical changes 

that occur as water moves through each aquifer. Sample 

PICR1 was left off of these plots due to its anomalous water 

quality and assumed lack of connectivity with other samples.

Carbon-14 Age Interpretation and Correction

Results for carbon-14 are reported in pmc, and mean 

residence times are calculated from the decay equation 

(Clark and Fritz, 1997). Samples reported as “modern” 

imply that recharge to groundwater occurred later than 1950 

(Woods Hole Oceanographic Institution, 2003). Uncorrected 

carbon-14 ages were calculated by the laboratory using the 

decay equation:

14 14

14

14

8267 ln( / )

where

is the age of the water sample, in years,

is the measured carbon-14 activity in, and

groundwater, and

is the carbon-14 activity in modern soil

(set as 100 pmc).

o

o

t aCa C

t

a C

a C

= − (2)

The decay equation does not account for carbonate 

dissolution. To account for potential carbonate dissolution 

and to obtain a more accurate residence time, carbon-14 

measurements were corrected using a δ13C mixing model 

(Clark and Fritz, 1997). The δ13C mixing model (developed 

by Pearson and White, 1967) is a useful tool for correcting 

carbon-14 age calculations; this model can reliably correct 

carbon-14 dilution caused by the large difference in δ13C 

between soil-derived dissolved inorganic carbon (DIC) and 

carbonate minerals in the aquifer. Hutchings and Petrich 

(2002) suggested that relatively young basin margin waters 

need to be corrected using an open-system carbon-13 mixing 

model. The δ13C mixing model-correction factor is calculated 

using the following equation:

13 13 13 13 13

DIC DIC carb rech carb

13

DIC

13 13

DIC

13 13

carb

C ( C C ) / ( C C )

where

C is the dilution factor,

C is the measured C of groundwater,

C is the C of calcite being dissolved

(assumed to be 0 ‰ by Hu

q

q

δ = δ −δ δ −δ

δ

δ δ

δ δ

13 13

rech

tchings

and Petrich [2002]), and 

C is the C of DIC in the infiltrating 

groundwaters, defined as:

δ δ

(3)
10 Recharge Sources and Residence Times of Groundwater in the Mayfield Area, Southwestern Idaho, 2011–12

( )

( )

2

2

13 13 13

rech soil DIC CO soil

13 13

soil 2

13

DIC CO soil

13

soil

C C C

where

C is the C of soil CO (assumed to 

be 23 ‰, as suggested by 

Hutchings and Petrich [2002]), and

C is the pH dependent enrichment 

C by 

−

−

δ =δ +ε

δ δ

−

ε

δ

2 soil

fractionation between

soil CO and DIC .

(4)

pH enrichment factors were calculated by an estimated 

fraction factor from Clark and Fritz (1997, table 5.2) based on 

sample temperature and are presented with carbon-14 data. 

Dilution factors were then calculated and multiplied by the 

uncorrected age to obtain a corrected age for carbon-14.

Carbon-14 ages are best understood as relative ages 

due to the uncertainties used in correcting carbon-14 values. 

Corrected ages are an estimate of the mean residence 

time of the water; however, many assumptions went into 

making calculations that introduce uncertainties to the age. 

Assumed values incorporate the carbon-13 values of soil and 

carbonate in the system, values could actually vary greatly 

from the numbers used in this model. Changing one of these 

carbon-13 end members could change the dilution factor and 

corrected age significantly. Additionally, other assumptions 

add uncertainty to this model. Atmospheric carbon-14 

activities have changed throughout time, but for simplification 

of calculations, this number was set to a static 100 pmc. 

Paleorecharge conditions may have been different than current 

conditions, which would greatly affect the fractionation factors 

and therefore the carbon-13 value of the sample; recharge 

conditions were assumed constant for the correction model. 

Due to the amount of assumptions involved, it is difficult to 

quantify the amount of uncertainty associated with a given 

carbon-14 value. One must remember that corrected carbon-14 

ages are most powerful when comparing multiple samples.

CFC Calibration Using Dissolved Gas Data

Analyses of select dissolved gases in groundwater (N2, 

Ar, CH4, O2, and CO2) provide gas concentrations and excess 

air concentrations, allowing for identification of recharge 

conditions and estimates of recharge temperature. Excess 

air concentrations and recharge temperature estimates are 

used in the analysis of CFC data to reduce uncertainty in 

the calculated recharge dates. Modeled recharge dates are 

based on the time that groundwater became isolated from 

the unsaturated zone and entered the aquifer (Plummer and 

others, 1993). 

Recharge altitudes used in temperature and CFC 

modeling are estimates of where recharge occurred. Because 

of the uncertainty associated with the altitude estimate, a 

sensitivity analysis was used to determine how variations 

in the recharge altitude would affect the CFC results. The 

minimum recharge altitude used was that of the well, and the 

maximum recharge altitude used was 6,700 ft (a high altitude 

in the Danskin Mountains). The range of recharge altitudes 

resulted in a maximum temperature difference of 5.8°C. The 

CFC model was recalibrated using the range of recharge 

temperatures and altitudes, resulting in a variation of less 

than 10 years in the recharge dates. Thus, changing recharge 

altitude and temperature did not affect the presence of CFCs 

and the interpretation related to their presence in groundwater.

Recharge dates were determined from CFC 

concentrations and were calibrated with recharge temperature 

and altitude information. If two different recharge 

temperatures were determined from dissolved gas samples, 

the average of the two temperatures was used in the CFC 

model. Five CFC samples were collected in the field at each 

site, but only 2–3 samples from each site were analyzed in 

the laboratory. 

Geochemical Modeling

The computer program NETPATH (Plummer and 

others, 1994) was used to simulate the change in chemical 

composition of groundwater as it moves along a flow path 

from an initial well (upgradient) to a final well (downgradient), 

and to calculate travel times using groundwater ages 

determined with corrected carbon-14 ages. NETPATH 

models were constrained by the mineralogy of the aquifer 

and mineral saturation indices. Models were limited to the 

elements carbon, calcium, sodium, magnesium, silica, fluoride, 

iron, potassium, aluminum, as well as by carbon-13 values. 

Mineral phases were limited to a very simple mineralogy 

including calcite, potassium feldspar, biotite, fluorite, calcium 

montmorillinite, CO2 gas, pyrite, SiO2, and plagioclase with 

45 percent anorthite composition. The mineral phases included 

in modeling efforts are representative of dominant mineralogy 

in the study area. Mineral phases were based on the presence 

of silicate minerals identified by Wood and Low (1988) and 

Welhan (2012). However, many other minerals could exist 

along the flow path; including additional minerals may have 

generated additional valid models. The model simulation 

results presented in this report are one representation of likely 

geochemical interactions between wells.

Carbon-13 values that were incorporated into flowpath models were taken from previous modeling efforts in 

the nearby Treasure Valley and assumed to be valid; the 

δ13C value of soil CO2 was assumed to be –23 ‰, and the 

δ13C value of dissolved calcite was assumed to be 0.0 ‰, as 

suggested by Hutchings and Petrich (2002). In NETPATH 

models, just as in carbon-14 models, there is much uncertainty 

when dealing with carbon-13 and carbon-14 values and 

unknowns. Flow paths were simulated from an upgradient 

well to a downgradient well, and from the surface-water 

sample to a downgradient well. The groundwater sample 
Concentration and Distribution of Geochemical Tracers 11

collected from the perched aquifer well near Indian Creek 

(PICR1) was not included in any of the simulations because 

its unique chemistry indicated that it was not a representative 

well for flow-path modeling efforts.

Mixing models were used to examine potential sources of 

recharge to the area. For these models, an upgradient well was 

mixed with an input water to result in the geochemistry found 

at the final well. Input sources to mixing models included 

another well or surface water (represented by the Indian Creek 

surface-water sample). In addition models were developed 

to incorporate the chemistry of geothermal water into the 

flow path. Water from an upgradient well was mixed with a 

chemical composition representative of a hot spring from the 

South Fork Payette geothermal system (Sacajawea Hot Spring) 

(Druschel and Rosenberg, 2000) or a chemical composition 

representative from the Boise Geothermal Aquifer (Berkeley 

Group, Inc., 1990). Geochemistry from these two hot springs 

is typical of geothermal water from different areas in the 

Idaho Batholith and may represent upwelling geothermal 

water; these mixing models are used to simulate the chemical 

evolution of water from the mixing of upgradient groundwater 

with geothermal water and mineral interaction within the 

aquifer. For models incorporating surface water, carbon-13 

was not used as a constraint due to unknown values from this 

input. Many invalid mixing models suggested the precipitation 

of carbon dioxide along a flow path; this was not considered 

a likely reaction due to low saturation indices in individual 

wells. Model simulation results included mineral reactions 

and percent contribution from each aquifer; if multiple models 

were possible, a range of contribution percentage from each 

aquifer was presented. 

Geochemical models were considered successful only 

if they produced a likely geochemical output. Any models 

that produced an unlikely geochemical reaction (such 

as precipitation of potassium feldspar) were considered 

unsuccessful and invalid models. In addition, modeled 

carbon isotope values were used to check the validity 

of the geochemical model. Modeled carbon-13 values 

were generated by mass balance and reflected the likely 

trend of isotopic evolution. Calculating carbon-13 values 

involved comparing the sensitivity of isotopic values at the 

downgradient well to the uncertainties associated with the 

isotopic data for the geochemical end members (carbon 

dioxide and calcite). Therefore, for a model to be considered 

valid, simulated carbon-13 values were required to match the 

observed values.

Cross Section Construction

Groundwater was assumed to flow to the southwest, 

approximately perpendicular from the range front. A cross 

section (A-Aꞌ, fig. 1) along an approximate flow path was 

constructed using well completion information and a cross 

section from Wellhan (2012). The cross section mimics one 

of the current hydrological conceptual models and allows 

display of geochemical changes with depth; another valid 

conceptual model shows the perched aquifer being laterally 

discontinuous. Flow paths from well to well within this cross 

section are consistent with water level data which indicate 

groundwater flows to the southwest; this conceptualization is 

consistent with that of Wellhan (2012).

Concentration and Distribution of 

Geochemical Tracers

Relations and spatial patterns in water quality that result 

from reactions of groundwater with aquifer matrix along a 

flow path and from mixing of different sources of recharge are 

often evident in physical parameters and major-ion chemistry. 

In this study, field parameters (table 3) varied between water 

samples, depending on which aquifer and at what depth the 

sample was collected. Field measurements of pH ranged 

from 6.6 to 8.1, with higher values in the deep aquifer. 

Table 3. Field parameter values for water samples collected in the 

Mayfield area, southwestern Idaho, 2011–12.

[Abbreviations: CaCO3, calcium carbonate; °C, degrees Celsius; μS/cm, 

microsiemens per centimeter; mg/L, milligram per liter; –, no data available]

Unique 

station 

identifier

pH

Specific 

conductance

(μS/cm)

Dissolved 

oxygen

(mg/L)

Water

temperature,

(°C) 

Alkalinity, 

(mg/L 

CaCO3

)

Perched aquifer near Indian Creek Reservoir

PICR1 7.1 1,120 5.85 13.0 277

Mountain system

M1 6.7 181 2.08 14.1 61.7

Perched aquifer

P1 6.7 218 7.85 13.6 62.5

P2 6.7 216 0.48 13.8 94.0

P3 6.9 212 6.38 14.3 80.9

P4 6.7 237 5.88 14.5 82.1

P5 6.9 205 7.87 15.3 74.2

Deep aquifer

D1 6.7 170 3.01 17.9 85.1

D2 6.9 172 1.09 21.0 74.9

D3 7.4 177 6.25 19.1 81.7

D4 8.1 220 4.33 24.2 99.5

D5 7.2 225 – 21.0 85.7

D6 8.1 221 6.91 25.9 83.9

D7 7.5 217 6.20 23.4 100

Surface water

SW1 6.6 84.0 14.02 0.2 45.0
12 Recharge Sources and Residence Times of Groundwater in the Mayfield Area, Southwestern Idaho, 2011–12

Specific conductance of the surface-water sample was low 

(84 μS/cm) as a result of recent snowmelt runoff (fig. 2), while 

groundwater generally exhibited a relatively narrow range 

of specific conductance between 170–225 μS/cm (table 3). 

The three most upgradient deep aquifer wells (D1-D3) had 

a narrow range of conductance values (170-177 μS/cm), 

which were the lowest that were measured in the groundwater 

network. The mountain front well (M1) also had a relatively 

low specific conductance value (181 μS/cm). Conductance 

values in the perched aquifer wells were higher, varied little 

(205-237 μS/cm) and were similar to values for wells D4-D7 

(217-225 μS/cm), the four most downgradient wells in the 

deep aquifer. The specific conductance of the sample collected 

from the perched aquifer near Indian Creek Reservoir was 

1,120 μS/cm, which was considerably higher than all of the 

other water samples and is considered an anomaly, relative 

to the regional groundwater system. Alkalinity values had a 

similar pattern to that of conductance, ranging from 45.0 to 

277 mg/L, with the lowest value in the surface water and 

Table 4. Major-ion concentrations in water samples collected from the Mayfield area, southwestern Idaho, 2011–12.

[Abbreviation: LT-MDL, long term-method detection limit]

Station name

Unique 

station 

identifier

Calcium, 

water,

filtered

(mg/L)

Magnesium, 

water,

filtered

(mg/L)

Sodium, 

water,

filtered

(mg/L)

Potassium, 

water,

filtered

(mg/L)

Chloride, 

water,

filtered

(mg/L)

Sulfate, 

water, 

filtered

(mg/L)

Fluoride, 

water,

filtered

(mg/L)

LT-MDL 0.022 0.011 0.060 0.030 0.060 0.090 0.040

Perched aquifer near Indian Creek Reservoir

01N 04E 29DCC1 PICR1 110 31 82 3.4 140 71 0.16

Mountain system

01N 05E 06CAB1 M1 17 4.5 9.7 1.7 4.6 6.0 0.13

Perched aquifer

01N 04E 27ACD2 P1 21 4.6 15 2.3 2.8 7.2 0.22

01N 04E 12CAC2 P2 19 5.6 16 1.5 4.0 7.6 0.29

01N 04E 13DAC1 P3 19 5.3 15 1.5 4.6 6.3 0.21

01N 04E 23ABA1 P4 21 5.4 17 1.8 6.5 10 0.20

01N 04E 4DDAD1 P5 18 5.0 14 1.8 5.6 6.5 0.30

Deep aquifer

01N 04E 13CBA1 D1 16 4.1 11 1.6 3.1 5.2 0.33

01N 04E 23ADC1 D2 14 3.6 18 1.5 2.3 8.4 0.23

01N 04E 27ACD1 D3 18 3.7 12 1.5 2.3 4.5 0.19

01N 04E 32AAB1 D4 18 2.9 24 1.5 3.1 7.6 0.38

02N 03E 34ACC1 D5 21 5.1 20 1.5 3.9 8.9 0.32

01S 04E 30ADC1 D6 14 2.0 28 5.2 7.6 12 0.51

01N 03E 11DDB1 D7 22 4.6 17 1.7 3.2 7.2 0.27

Surface water

Indian Creek near 

Mayfield, Idaho

SW1 8.1 1.7 7.5 1.9 1.5 3.1 0.11

Replicate samples

01N 05E 06CAB1 M1R 17 4.6 9.7 1.7 9.7 5.9 0.14

01N 03E 11DDB1 D7R 21 4.4 18 1.7 3.1 7.1 0.28

the highest values in the sample from the perched aquifer 

near Indian Creek Reservoir. Water temperatures in the 

perched aquifer ranged from 13.6 to 15.3°C, whereas higher 

temperatures (17.9 to 25.9°C) were measured in the deep 

aquifer. Warmer temperatures in the deeper wells may be the 

result of geothermal upwelling of warm water to the deep 

aquifer, or may simply be a product of the natural geothermal 

gradient (Welhan, 2012).

Major ion data (table 4) are used to characterize water 

types which help to understand the geochemical evolution 

of groundwater within an aquifer or along a flow path. 

Water from 13 of the 14 wells sampled was a mixed-cation 

bicarbonate water, with calcium and sodium being the 

co-dominant cations. Bicarbonate was the dominant anion for 

all wells. The sodium bicarbonate water type in well D6 was 

distinctly different from any other wells sampled. There was 

no consistent evolution of the major-ion chemistry along either 

the shallow or deep flow paths.
Concentration and Distribution of Geochemical Tracers 13

Many constituents did not show a one-directional change 

along the perched aquifer simulated flow path, suggesting that 

perched aquifer wells may not be hydrologically connected. 

Chemistry may also vary with depth, or be influenced by 

compartmentalization; however, major ion chemistry does 

not to fully explain this trend. A more complex flow path 

could exist, or wells could be separated by confining units 

and faulting. An alternate conceptual model of the study area 

suggests that the perched aquifer is laterally discontinuous; 

this type of aquifer would create compartmentalized and 

geochemically unique water and could explain variations seen 

in the chemistry of the perched aquifer.

The concentration of major ions variably increased or 

decreased along the relative flow path in the deep aquifer. 

As water moved downgradient in the deep aquifer, major ion 

concentrations varied, indicating that another type of water 

was being introduced (fig. 3). Samples from three deep aquifer 

0 

5 

10 

15 

20 

25 

0 

10 

20 

30 

0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

0 

1 

2 

3 

4 

5 

6 

0 

2 

4 

6 

8 

10 

12 

14 

0 

20 

40 

60 

80 

100 

120 

M1 P2 P3 P5 P4 P1 D3 D6 D7 D2D1 D4 D5

M1 P2 P3 P5 P4 P1 D3 D6 D7 D2D1 D4 D5

0 

1 

2 

3 

4 

5 

6 

0 

0.1 

0.2 

0.3 

0.4 

0.5 

0.6 

Calcium, in milligrams per liter Magnesium, in milligrams per liter Chloride, in milligrams per liter

As calcium carbonate,

in milligrams per liter

Fluoride, in milligrams per liter Sulfate, in milligrams per liter Potassium, in milligrams per liter

Sodium,

in milligrams per liter

Unique station identifier

Unique station identifier

Calcium

Magnesium

Chloride

Alkalinity

Sodium

Potassium

Sulfate

Fluoride

Mountain system

Perched aquifer

Deep aquifer 

EXPLANATION

Figure 3. Major ion concentrations along a relative flow path in water samples 

collected in wells from the Mayfield area, southwestern Idaho, 2011–12. Results from 

the perched aquifer near Indian Creek Reservoir are not included.
14 Recharge Sources and Residence Times of Groundwater in the Mayfield Area, Southwestern Idaho, 2011–12

Table 5. Nutrient concentrations in water samples collected in the Mayfield area, 

southwestern Idaho, 2011–12.

[Abbreviations: LT-MDL, long term-method detection limit; mg/L, <, less than; –, no data available]

Unique 

station 

identifier

 Nitrite, 

water, filtered 

(mg/L as 

nitrogen)

Nitrate, 

water, filtered 

(mg/L as 

nitrogen)

 Nitrate plus 

nitrite, 

water, filtered 

(mg/L as 

nitrogen)

Orthophosphate, 

water, filtered 

(mg/L as 

phosphorus)

Ammonia, 

water, filtered 

(mg/L as 

nitrogen)

LT-MDL 0.001 0.040 0.040 0.0040 0.010

Perched aquifer near Indian Creek Reservoir

PICR1 < 0.001 15 15 0.083 < 0.010

Mountain system

M1 < 0.001 3.1 3.1 0.13 < 0.010

Perched aquifer

P1 < 0.001 7.4 7.4 0.26 < 0.010

P2 < 0.001 0.50 0.50 0.27 < 0.010

P3 0.002 3.0 3.0 0.12 < 0.010

P4 < 0.001 4.2 4.2 – < 0.010

P5 < 0.001 3.1 3.1 0.14 < 0.010

Deep aquifer

D1 < 0.001 0.80 0.80 0.15 < 0.010

D2 < 0.007 0.14 0.15 0.12 0.032

D3 < 0.001 0.21 0.21 0.073 < 0.010

D4 < 0.001 0.11 0.11 0.020 < 0.010

D5 < 0.001 1.7 1.7 0.048 < 0.010

D6 < 0.001 0.64 0.64 0.011 0.020

D7 < 0.001 0.46 0.46 0.032 0.018

Surface water

SW1 < 0.001 0.09 0.09 0.066 0.027

Replicate samples

M1R < 0.001 3.0 3.0 0.13 < 0.010

D7R < 0.001 0.49 0.49 0.031 0.018

wells (D1, D2, and D3) had a 

lower specific conductivity than 

the mountain system sample or 

other deep aquifer samples. A 

change in chemistry is apparent 

between points D3 and D4; major 

ion patterns change between these 

two points in most flow-path cross 

plots, suggesting that a chemical 

change happens between these 

two wells (fig. 3; table 2). The 

variable concentrations of solutes 

in a downgradient direction may 

be partly due to mixing with 

geothermal water, or addition 

or removal of solutes to the 

solution from mineral dissolution 

or precipitation.

The water sample from the 

perched aquifer near Indian Creek 

Reservoir displayed an anomalous 

chemistry that was characterized 

by much higher concentrations 

of major ions and metals than 

of other water samples from all 

other wells. This perched aquifer 

well may reflect the influence 

of local surface-water recharge, 

evaporation, localized groundwater 

mounding, or human activity 

associated with septic systems and 

operations at a nearby commercial 

development; therefore, this 

sample is not representative of 

groundwater in the area, was not 

plotted in figure 3, and will not be 

included in further discussions.

Elevated concentrations of 

nutrients (nitrogen and phosphorus 

species) greater than normal background levels can indicate 

the presence of water affected by human activities such as 

agricultural fertilizer application or wastewater disposal 

(Hallberg and Keeny, 1993). Identifying the sources of 

elevated nutrient concentrations in groundwater is important 

because high concentrations may adversely affect the 

environment and human health. Nutrient concentrations were 

variable in the study area (table 5); generally, they were higher 

in the perched aquifer than in the deep aquifer, the mountain 

system, or Indian Creek. Higher nutrient concentrations in the 

perched aquifer suggest human activity influenced the source 

of recharge to this aquifer. Nutrient concentrations were 

generally lower in the deep aquifer than in the perched aquifer 

or mountain system, suggesting that infiltration of highnutrient water may not be contributing to the deep aquifer or 

that the deep aquifer has been diluted with low-nutrient water. 

The lag time for recharge to infiltrate into the deep aquifer 

may also account for this difference in nutrient concentration. 
Concentration and Distribution of Geochemical Tracers 15

Figure 4. Nitrate concentration compared with carbon-14 in water samples collected from the 

Mayfield area, southwestern Idaho, 2011–12.

tac13-0813_fig04

0 

2 

4 

6 

8 

10 

12 

14 

16 

0 20 40 60 80 100 120 

Nitrate as nitrogen, in milligrams per liter 

Carbon-14 value, in percent modern carbon 

Mountain system and 

unique identifier

Perched aquifer and 

unique identifier

Deep aquifer and 

unique identifier

Perched aquifer near 

Indian Creek Reservoir 

EXPLANATION

M1

P2

P3 P5

P4

P1

PICR1

D3

D6 D7

D2

D1

D4

D5

M1

P5

D6

PICR1

Nutrient data plotted with carbon-14 values indicate modern, 

higher-nutrient water is recharging the perched aquifer, 

whereas older, lower-nutrient water generally predominates 

in the deep aquifer (fig. 4). Recent recharge with higher 

concentrations of nutrients may not have reached the deep 

aquifer system at the time of sampling.

Naturally occurring metals and trace elements can be 

derived from aquifer material and transported in groundwater. 

Concentrations of metals and trace elements were variable 

in water samples collected from the study area (table 6). 

Higher concentrations of aluminum, beryllium, cobalt, copper, 

lead, iron, manganese, and nickel were measured in the 

Indian Creek surface-water sample than in the groundwater 

samples. Water from the deep aquifer generally had higher 

concentrations of lithium, molybdenum, and tungsten than 

water in the perched aquifer. Several deep aquifer wells and 

the mountain system well samples contained more elevated 

concentrations of some trace elements than the perched 

aquifer, such as lithium and arsenic (fig. 5), suggesting that 

sediment from the Danskin Mountains influences chemistry 

in the deep aquifer. SPF Engineering (2007b) also noted 

increasing arsenic concentrations with depth. Metals and trace 

elements did not show a consistent change in concentration 

along a relative flow path; some metals increased in 

concentration along the flow path, while others decreased or 

deviated from the trend at a given well (fig. 5).

The ratios of oxygen-18 and deuterium (hydrogen-2) 

in precipitation vary in response to the source of the 

precipitation, season, altitude, temperature, amount of 

evaporation, and other factors such as mixing of water from 

different sources. Thus, oxygen-18 and deuterium can provide 

information regarding the seasonality and altitude of the 

precipitation that recharged the groundwater. The ratios of 

oxygen-18 and deuterium to their lighter isotopes (also known 

as the signature) can indicate sources of recharge water and 

changes to water since precipitation, such as evaporation 

or water-rock interaction (Coplen, 1993). Meteoric water 

lines, such as the Global Meteoric Water Line (GMWL; 

Craig, 1961) or a Local Meteoric Water Line (LMWL), are 

used to describe a continuum in the isotopic signature of the 

precipitation. For this study, three meteoric water lines were 

plotted with data: the GMWL (δ2H= 8×δ18O + 10), a LMWL 

developed by Schlegel and others (2009) for the Boise area 

(δ2H= 6.94×δ18O – 10.7), and a LMWL developed by Wood 

and Low (1988) for the SRP area (δ2H= 6.4×δ18O – 21).
16 Recharge Sources and Residence Times of Groundwater in the Mayfield Area, Southwestern Idaho, 2011–12

Table 6. Metals and trace-element concentrations for water samples collected in the Mayfield area, 

southwestern Idaho, 2011–12.

[Abbreviations: LT-MDL, long term-method detection limit; μg/L, microgram per liter; <, less than]

Unique 

station 

identifier

 Aluminum, 

water, 

filtered

(μg/L)

Antimony, 

water, 

filtered

(μg/L)

Arsenic, 

water, 

filtered

(μg/L)

Barium, 

water, 

filtered

(μg/L)

Beryllium, 

water, 

filtered

(μg/L)

 Boron, 

water, 

filtered

(μg/L)

Cadmium, 

water, 

filtered

(μg/L)

Chromium, 

water, 

filtered

(μg/L)

LT-MDL 2.2 0.027 0.03 0.07 0.006 3 0.016 0.070

Perched aquifer near Indian Creek Reservoir

PICR1 < 1.7 0.057 2.9 280 < 0.006 56 < 0.016 0.25

Mountain system

M1 < 1.7 0.057 7.5 8.2 0.008 19 < 0.016 0.21

Perched aquifer

P1 20.7 0.087 3.2 42 < 0.006 25 < 0.016 1.2

P2 < 1.7 0.050 3.5 47 0.009 21 < 0.016 0.16

P3 < 1.7 0.049 1.8 49 < 0.006 15 < 0.016 0.53

P4 < 1.7 0.060 2.3 51 0.009 22 < 0.016 0.34

P5 < 1.7 0.056 2.4 45 < 0.006 17 < 0.016 0.68

Deep aquifer

D1 < 1.7 0.042 2.8 4.0 < 0.006 15 < 0.016 0.88

D2 14.6 0.074 5.5 5.6 < 0.006 14 < 0.016 0.28

D3 < 1.7 0.028 5.8 15 < 0.006 12 < 0.016 1.2

D4 < 1.7 0.029 2.5 4.6 < 0.006 18 < 0.016 0.38

D5 5.8 0.036 2.9 42 < 0.006 18 < 0.016 1.5

D6 2.2 0.027 6.8 15 < 0.006 46 < 0.016 9.9

D7 2.2 0.035 6.9 36 < 0.006 15 < 0.016 1.5

Surface water

SW1 350 0.11 1.6 18 0.04 15 < 0.016 0.25

Replicate samples

M1R <1.7 0.049 7.5 8.2 0.013 19 <0.016 0.20

D7R 2.2 0.036 6.9 36 <0.006 14 <0.016 1.4
Concentration and Distribution of Geochemical Tracers 17

Table 6. Metals and trace-element concentrations for water samples collected in the Mayfield area, 

southwestern Idaho, 2011–12.—Continued

[Abbreviations: LT-MDL, long term-method detection limit; μg/L, microgram per liter; <, less than]

Unique

station 

identifier

 Cobalt, 

water, 

filtered

(μg/L)

 Copper, 

water, 

filtered

(μg/L)

Iron, 

water, 

filtered

(μg/L)

 Lead, 

water, 

filtered

(μg/L)

Lithium, 

water, 

filtered

(μg/L)

Manganese, 

water, 

filtered

(μg/L)

Molybdenum, 

water, 

filtered

(μg/L)

 Nickel, 

water, 

filtered

(μg/L)

LT-MDL 0.021 0.80 3.2 0.025 0.22 0.13 0.014 0.09

Perched aquifer near Indian Creek Reservoir

PICR1 0.35 0.88 < 3.2 0.16 16 0.27 0.57 7.2

Mountain system

M1 0.026 1.9 < 3.2 0.087 7.7 0.31 0.34 < 0.09

Perched aquifer

P1 0.12 < 0.80 20 < 0.025 18 0.53 1.04 1.4

P2 < 0.020 < 0.50 28 < 0.025 4.8 3.51 1.0 0.3

P3 0.024 0.51 22 2.5 4.0 3.77 1.9 0.13

P4 0.031 1.1 < 3.2 0.10 7.4 0.83 0.90 0.22

P5 < 0.020 < 0.50 < 3.2 0.075 4.7 < 0.13 0.81 0.09

Deep aquifer

D1 < 0.020 < 0.50 14 0.025 12 1.23 1.5 0.12

D2 0.15 < 0.80 7.2 0.052 17 18 2.6 0.37

D3 0.021 < 0.50 < 3.2 0.018 14 0.19 1.2 < 0.090

D4 < 0.020 1.1 6.8 0.41 16 0.16 1.5 < 0.090

D5 0.14 < 0.8 49 0.34 9.2 8.12 1.7 0.24

D6 < 0.021 < 0.8 4.3 0.026 11 0.66 3.1 < 0.090

D7 < 0.021 6.1 < 3.2 0.092 14 0.17 1.6 0.20

Surface water

SW1 3.3 2.5 498 1.48 16.7 34.3 0.64 0.87

Replicate samples

M1R < 0.020 1.8 < 3.2 0.085 7.7 0.30 0.35 0.090

D7R < 0.021 3.2 < 3.2 0.091 12 0.18 1.7 0.19
18 Recharge Sources and Residence Times of Groundwater in the Mayfield Area, Southwestern Idaho, 2011–12

Table 6. Metals and trace-element concentrations for water samples collected in the Mayfield area, 

southwestern Idaho, 2011–12.—Continued

[Abbreviations: LT-MDL, long term-method detection limit; μg/L, microgram per liter; <, less than]

Unique 

station 

identifier

Selenium, 

water, 

filtered

(μg/L)

Silica, 

water, 

filtered

(μg/L as

SiO2)

 Silver, 

water, 

filtered

(μg/L)

Strontium, 

water, 

filtered

(μg/L)

Thallium, 

water, 

filtered

(μg/L)

 Tungsten, 

water, 

filtered

(μg/L)

Vanadium, 

water, 

filtered

(μg/L)

 Uranium 

(natural), 

water, 

filtered

(μg/L)

 Zinc, 

water, 

filtered

(μg/L)

LT-MDL 0.030 0.060 0.005 0.200 0.01 0.010 0.080 0.0040 1.4

Perched aquifer near Indian Creek Reservoir

PICR1 0.77 48 < 0.005 743 < 0.01 0.018 9.0 27 18

Mountain system

M1 0.14 43 < 0.005 133 < 0.01 <0.010 2.6 0.49 9.0

Perched aquifer

P1 1.0 44 < 0.005 159 < 0.01 0.027 4.9 0.12 2.8

P2 0.050 53 < 0.005 142 < 0.01 < 0.010 4.2 0.34 5.7

P3 0.21 47 < 0.005 134 < 0.01 < 0.010 6.7 0.36 140

P4 0.54 39 < 0.005 173 < 0.01 0.016 4.5 0.66 < 1.4

P5 0.34 50 < 0.005 129 < 0.01 < 0.010 6.1 0.47 9.7

Deep aquifer

D1 1.0 54 < 0.005 106 < 0.01 < 0.010 2.1 0.26 190

D2 0.22 47 < 0.005 82.1 < 0.01 0.14 2.0 0.33 3.6

D3 0.52 46 < 0.005 103 < 0.01 0.013 2.5 0.29 1.8

D4 0.11 35 < 0.005 95.5 < 0.01 0.20 5.0 0.29 18

D5 0.47 42 < 0.005 160 < 0.01 0.24 4.7 2.9 80

D6 0.36 63 < 0.005 66.6 < 0.01 0.82 35 0.57 23

D7 0.41 48 < 0.005 140 < 0.01 0.096 6.1 1.1 6.0

Surface water

SW1 0.070 30 < 0.005 53.8 < 0.01 0.011 2.4 0.37 5.3

Replicate samples

M1R 0.13 42 < 0.005 132 < 0.01 <0.010 2.6 0.49 9.2

D7R 0.41 48 < 0.005 141 < 0.01 0.097 6 1.1 5.9
Concentration and Distribution of Geochemical Tracers 19

tac13-0813_fig05

0 

1 

2 

3 

4 

5 

6 

7 

8 

0 

2 

4 

6 

8 

10 

12 

14 

16 

18 

20 

Arsenic, in micrograms per liter Lithium, in micrograms per liter

Arsenic

Lithium

Mountain system

Perched aquifer

Deep aquifer

Indian Creek

EXPLANATION

M1 SW1 P2 P3 P5 P4 P1 D3 D6 D7 D2D1 D4 D5

Unique station identifier

Figure 5. Concentrations of arsenic and lithium along a relative flow path 

in water samples collected from wells from the Mayfield area, southwestern 

Idaho, 2011–12.
20 Recharge Sources and Residence Times of Groundwater in the Mayfield Area, Southwestern Idaho, 2011–12

Table 7. Isotope data and corrected carbon-14 ages for water samples collected in the Mayfield area, southwestern Idaho, 2011–12. 

[“Modern,” indicates water recharged since the 1950s. Abbreviations: ‰, per mil; δ2H H2O, hydrogen-2 in water; δ18O H2O, oxygen-18 in water; ‰, per mil; 

δ13C, carbon-13; pmc, percent modern carbon; ɛ13CDIC-CO2(soil), pH dependent enrichment factor; N/A, not applicable]

Unique 

station 

identifier

δ2

H H2O 

(‰)

δ18O H2O 

(‰)

δ13C 

(‰)

14C uncorrected 

(pmc)

14C error 

(pmc)

pH enrichment 

factor 

(ɛ13CDIC-CO2

(soil))

Dilution 

factor

Uncorrected age

(years)

Corrected 

age

(years)

PICR1 -116.99 -14.72 -15.38 93.54 0.28 18.3 -0.84 552 Modern

M1 -120.98 -15.18 -17.15 102.90 0.43 -14 1.2 Modern Modern

P1 -121.04 -15.30 -16.42 104.40 0.40 -11 1.5 Modern Modern

P2 -124.85 -15.68 -16.46 90.40 0.31 -9.7 1.7 834 1,400

P3 -124.38 -15.43 -15.15 93.76 0.29 -11 1.3 533 710

P4 -120.55 -15.16 -15.53 111.30 0.44 -12 1.4 Modern Modern

P5 -124.63 -15.72 -14.33 96.00 0.29 -13 1.2 337 390

D1 -125.25 -15.64 -15.06 74.81 0.23 -8.2 1.1 2,399 2,700

D2 -125.36 -15.93 -15.37 74.51 0.23 -7.7 1.2 2,432 2,900

D3 -126.10 -16.02 -14.01 67.88 0.24 -9.3 0.98 3,203 3,100

D4 -124.76 -15.61 -12.96 40.07 0.15 -4.7 1.3 7,561 10,000

D5 -127.02 -16.10 -13.34 55.34 0.17 -4.8 1.4 4,891 6,700

D6 -132.01 -16.68 -11.94 32.56 0.13 -6.8 1.0 9,276 9,400

D7 -125.74 -15.84 -14.13 54.77 0.18 -5.3 1.4 4,977 6,800

SW1 -133.37 -17.22 N/A N/A N/A N/A N/A N/A N/A

M1R -121.12 -15.21 -17.25 101.56 0.33 N/A N/A N/A N/A

D7R -126.02 -15.90 -14.13 55.24 0.20 N/A N/A N/A N/A

The stable isotope ratios in water samples (table 7) were 

plotted on a delta diagram (fig. 6) with the GMWL and the 

two LMWLs, as well as the mean stable isotope values from 

geothermal waters in Idaho (Parliman and Young, 1992). The 

most negative (depleted) ratio was from the Indian Creek 

sample collected during winter. Samples from the perched 

aquifers tended to be more enriched (less negative ratios) than 

water from the deep aquifer. All samples collected during this 

study were more enriched than a data point representing mean 

geothermal water in Idaho, but still are within the range of 

stable isotopes exhibited by geothermal water (Parliman and 

Young, 1992). All water samples plot below the GMWL and 

LMWLs, but they still fall in a linear pattern, likely indicating 

that the study area may have its own meteoric water line 

different from the GMWL and the LMWLs. However, if the 

LMWLs actually represent local recharge, water samples that 

plot below and to the right of the LMWLs indicate enrichment 

of oxygen-18 as compared to deuterium. Preferential 

enrichment of oxygen-18 can be caused by evaporation (which 

generates a slope between 3 and 6, less than the slope of data 

for this study) or water-rock interaction (which increases only 

the oxygen-18 signature) (Clark and Fritz, 1997). Based on 

geochemical modeling results, a large amount of water-rock 

interaction is probably not occurring; therefore, this pattern 

is likely caused by a different LMWL than what has been 

reported for nearby areas.

The most commonly used method of dating groundwater 

with an age of recharge between 1,000 and 40,000 years 

is carbon-14 (half-life of 5,370 years) dating. Carbon in 

groundwater is derived from various components of the 

carbon cycle, and carbon isotopes provide information about 

interactions between groundwater and its environment. 
Concentration and Distribution of Geochemical Tracers 21

Figure 6. Oxygen and hydrogen isotopes from groundwater and surface water in the Mayfield area, 

southwestern Idaho, 2011–12, plotted with a global meteoric water line (GMWL) and two local meteoric 

water lines (LMWLs).

tac13-0813_fig06

EXPLANATION

-145 

-140 

-135 

-130 

-125 

-120 

-115 

-110 

-18 -17.5 -17 -16.5 -16 -15.5 -15 -14.5 -14 

Hydrogen-2 (Vienna Standard Mean Ocean Water) 

Oxygen-18 (Vienna Standard Mean Ocean Water) 

y = 8x + 10 y = 6.94x - 10.7 

y = 6.4x - 21 

M1

SW1

P5 P2 P3

P4

P1

D3

D6

M1

P5

D6

SW1

D7 D2

D1

D4

D5

PICR1

PICR1

Linear (Global Meteoric Water Line)

Linear (Local Meteoric Water Line [Boise area])

Linear (Local Meteoric Water Line [Snake River Plain])

Perched aquifer near Indian Creek Reservoir and 

unique identifier

Mountain system and unique identifier

Perched aquifer and unique identifier

Deep aquifer and unique identifier

Indian Creek–January and unique identifier

Mean of Idaho geothermal water 

(Parlamin and Young, 1992)
22 Recharge Sources and Residence Times of Groundwater in the Mayfield Area, Southwestern Idaho, 2011–12

Most of the carbon-14 measured in groundwater is derived 

from carbon dioxide or inorganic carbon present in the soil 

zone during recharge that dissolves into water as it moves 

through the saturated zone. As water moves along a flow path, 

carbon-14 can be diluted, necessitating correction techniques 

that can range from simple mathematical calculations 

to complex geochemical modeling. For this study, an 

open‐system carbon-13 mixing model was applied to evaluate 

whether significant carbonate dissolution occurred along the 

flow path (Coplen, 1993; Clark and Fritz, 1997). Estimated 

carbon-13 values were used in this model and corrected ages 

contain a large amount of uncertainty.

Carbon-13 ratios, carbon-14 results, and interpretations 

of age dating are presented in table 7. Carbon-13 ratios were 

most depleted (negative) in the mountain system and in the 

upgradient part of the deep and perched aquifers, and more 

enriched (less negative) farther down the flow path. Carbon-14 

corrected ages in samples collected from the perched aquifer 

ranged from modern (less than 100 years old) to 1,400 years 

old; the sample from the mountain system also contained 

modern water. However, corrected carbon-14 ages in samples 

collected from the deep aquifer ranged from 2,700 to 

10,000 years old. Carbon-14 age dates in the deep aquifer 

generally increased with depth and distance along a flow path, 

indicating that the conceptual flow path in the deep aquifer is 

reasonable. Age dates in the perched aquifer tended to increase 

with depth but not with distance from the mountain front, 

perhaps suggesting that the perched aquifer is discontinuous. 

Dissolved gases were used to determine the recharge 

temperature and to calibrate CFC models. Separate analyses 

were performed for each sample for quality-control purposes 

(table 8). Estimated recharge temperatures ranged from 5.7 to 

18.9°C, and are reported based on recharge 500 ft above 

the land-surface at the well (table 8) and were used in CFC 

calculations. 

Chlorofluorocarbons are stable volatile organic 

compounds (VOCs) that were introduced to the atmosphere 

beginning in the 1930s during production of refrigerants, 

cleaning solvents, foams, plastic, and other synthetic 

materials (Clark and Fritz, 1997). CFCs released into the 

atmosphere are stable; historical atmospheric concentrations 

of CFCs are known. Measuring ratios of CFC-12 

(dichlorodifluoromethane), CFC-11 (trichlorofluoromethane), 

and CFC-113 (trichlorotrifluorethane) in groundwater enables 

identification of the age of groundwater recharged since about 

1941, 1947, and 1955, respectively. Laboratory results include 

the amount of CFCs in a given sample, a modeled recharge 

date, and the amount of young water as determined by CFCs 

in a sample. 

CFCs were found in samples from every well in the 

study area, indicating all groundwater samples contained some 

modern recharge (table 9). Modeled recharge dates ranged 

from the late 1940s to the late 1980s. Recharge dates for 

samples in the perched aquifer ranged from the late 1940s to 

the mid-1980s, whereas samples in the deep aquifer ranged 

from the late 1940s to the mid-1970s. Samples did not show 

a systematic pattern in recharge date along a flow path. CFC 

results also included an estimate of the amount of young 

water (water dateable by CFCs) in each sample by ratio dating 

(table 9). Samples in the Mayfield area contained between 1 

and 94 percent young water, with the smallest percentage of 

young water in P2 (between 1 and 6 percent young water) and 

D2 (between 2 and 7 percent young water) (fig. 7).

The age tracers used in this study (carbon-14 and CFCs) 

produced different recharge dates for the same sample of 

water. Carbon-14 dates indicated that groundwater ages ranged 

from modern to thousands of years; all samples from the deep 

aquifer had mean residence times of more than 2,000 years. 

Modern recharge (indicated by the presence of CFCs) was 

present in every water sample collected from wells, including 

those wells with older water dated by carbon-14. Older water 

could be a product of depth, distance from the recharge 

zone, or isolation due to a local confining unit; however, the 

presence of CFCs in water with an older mean residence 

time indicates that some modern recharge is making its way 

to the deeper part of the aquifer. The presence of both young 

and old water suggests that the study area contains mixed 

groundwater, or that groundwater is not moving as piston flow, 

a basic assumption used in the computation of CFC-derived 

recharge dates. 

Geochemical modeling results refine concepts of flow 

paths and sources of recharge. Model simulation results can 

be confirmed by calculated saturation indices for individual 

samples (table 10). Mineral phases that were included in 

models were constrained by mineral reactions along the flow 

path; successful models showed the dissolution of biotite, 

calcite, CO2 gas, fluorite, plagioclase, potassium feldspar, and 

pyrite and the precipitation of calcium montmorollionite, ferric 

hydroxide, and SiO2 (table 11). Many attempted flow paths 

were considered invalid because they showed precipitation of 

CO2 gas, an unlikely reaction in this area.
Concentration and Distribution of Geochemical Tracers 23

Table 8. Dissolved gas data and estimated recharge temperatures for water samples collected in the Mayfield 

area, southwestern Idaho, 2011–12.

[Samples analyzed in duplicate. Abbreviations: N2, nitrogen gas; Ar, argon gas; O2, oxygen gas; CO2, carbon dioxide gas; CH4, methane 

gas; mg/L, milligram per liter; ft, foot; °C, degrees Celsius; cc STP/L, cubic centimeters per liter of pressure at standard temperature and 

pressure]

Unique

station 

identifier

N2

(mg/L)

Ar

(mg/L)

O2

(mg/L)

CO2

(mg/L)

CH4

(mg/L)

Assigned 

recharge 

altitude

(ft)

Recommended 

recharge 

temperature

(°C)

Excess air 

(cc STP/L)

PICR1 19.7967 0.6816 4.9022 42.0630 0.00000 3,859 8.09 3.49

PICR1 19.5972 0.6774 4.9443 42.1243 0.00000 3,859 8.19 3.32

M1 20.0588 0.6846 1.7664 32.8804 0.00000 4,261 7.68 3.85

M1 20.0297 0.6834 1.7996 32.7183 0.00000 4,261 7.76 3.85

P1 19.5594 0.6987 4.1333 29.3521 0.00000 3,929 5.71 2.34

P1 19.3694 0.6926 3.8675 27.9613 0.00000 3,929 6.01 2.27

P2 20.0106 0.6758 0.4896 34.5638 0.00000 4,082 8.87 4.14

P2 19.8813 0.6721 0.4489 34.7401 0.00000 4,082 9.04 4.07

P3 19.8028 0.6654 5.5482 25.6374 0.00092 4,071 9.71 4.22

P3 19.5926 0.6613 5.6115 25.5677 0.00088 4,071 9.76 4.02

P4 18.6033 0.6786 5.3373 30.4024 0.00000 4,017 5.99 1.54

P4 18.5319 0.6756 5.2976 30.2968 0.00000 4,017 6.18 1.54

P5 15.5559 0.5873 6.9866 15.7585 0.00000 4,030 10.5 0.12

P5 15.3666 0.5856 6.6045 16.4499 0.00000 4,030 10.3 -0.14

D1 17.4287 0.6224 2.2436 18.2853 0.00000 4,067 10.0 1.90

D1 17.3785 0.6213 2.3615 18.4592 0.00000 4,067 10.1 1.86

D2 16.8221 0.6172 0.2204 15.4252 0.00000 3,983 9.54 1.05

D2 16.7620 0.6140 0.2077 15.7818 0.00000 3,983 9.81 1.09

D3 14.3083 0.5360 5.2403 8.59014 0.00000 3,929 14.8 0.13

D3 14.4343 0.5398 5.0789 8.61414 0.00000 3,929 14.5 0.19

D4 14.5205 0.5193 3.8073 2.13068 0.00000 3,870 18.0 1.22

D4 14.3650 0.5147 3.6915 2.14732 0.00000 3,870 18.4 1.15

D5 14.7266 0.5170 2.9672 8.78133 0.00000 3,917 18.9 1.68

D5 14.8346 0.5189 2.5419 9.07804 0.00000 3,917 18.9 1.79

D6 16.9178 0.5890 4.0311 1.28358 0.00000 3,649 14.0 2.41

D6 16.8019 0.5864 4.0852 1.22602 0.00000 3,649 14.1 2.32

D7 13.5283 0.4994 1.3913 10.3746 0.00000 3,855 18.6 0.34

D7, 13.5281 0.4984 2.2776 9.62868 0.00000 3,855 18.8 0.38
24 Recharge Sources and Residence Times of Groundwater in the Mayfield Area, Southwestern Idaho, 2011–12

Table 9. Chlorofluorocarbon results and simulated groundwater recharge dates based on recharge elevation for groundwater 

samples collected in the Mayfield area, southwestern Idaho, 2011–12.

[Samples analyzed in duplicate or triplicate. Sample identified by station name or well identifier. Abbreviations: pmol/kg, picomole per kilogram; 

°C, degrees Celsius; ft, foot; ERR, error; NP, not possible to determine]

Unique 

station 

identifier

Corrected concentrations 

(pmol/kg)

Calculated 

recharge 

temperature

(°C)

Assumed 

recharge 

elevation

(ft)

Young 

water in 

sample 

(percent)

Model piston dates

(excess air corrected) Recommended 

age based on

Young fraction

recharge date

CFC-11 CFC-12 CFC-113 CFC-11 CFC-12 CFC-113

Perched aquifer near Indian Creek Reservoir

PICR1 2.11 5.19 0.334 8.15 3,859 92–94 1989 1985.5 1986.5 CFC-11,12,113 Mid to late1980s

PICR1 2.15 4.96 0.342 8.15 3,859 1987.5 1985.5 1986.5

PICR3 2.09 5.16 0.338 8.15 3,859 1988.5 1985.5 1986.5

Mountain system

M1 0.915 1.09 0.144 7.6 4,261 20–52 1969 1972 1979.5 CFC-11,12 Late 1960s to early 1970s

M1 0.921 1.07 0.117 7.6 4,261 1969 1972.5 1978

M1 0.908 1.12 0.105 7.6 4,261 1969.5 1972 1977

Perched aquifer

P1 2.47 4.08 0.318 5.85 3,929 76–77 1981 1986.5 1985 CFC-12,113 Mid 1980s

P1 2.45 4.06 0.319 5.85 3,929 1981 1986 1985 NP

P2 0.0417 0.0733 0.008 8.95 4,082 1–6 1955 1950.5 1959.5 CFC-11,12,113 Late 1940s to early 1950s

P2 0.0312 0.0732 0.007 8.95 4,082 1955 1949 1958.5

P2 0.0253 0.0682 0.011 8.95 4,082 1954.5 1948 1961.5

P3 0.0473 0.100 0.006 9.75 4,071 3–17 1956.5 1951 1958 CFC-11,12,113 Early to mid 1950s

P3 0.0655 0.100 0.007 9.75 4,071 1956.5 1953 1959

P3 0.0603 0.111 0.008 9.75 4,071 1956.5 1952.5 1960

P4 1.54 2.86 0.204 6.10 4,057 60–78 1975.5 1977 1981.5 CFC-11,12,113 Mid to late 1970s

P4 1.55 2.87 0.205 6.10 4,057 1975.5 1977 1981.5

P4 1.57 2.93 0.208 6.10 4,057 1976 1978 1981.5

P5 1.03 1.50 0.158 10.4 4,030 34–57 1972.5 1975 1982 CFC-11,12 Early to mid 1970s

P5 1.06 1.55 0.159 10.4 4,030 1972.5 1975 1982

P5 1.05 1.52 0.156 10.4 4,030 1972.5 1975 1981.5

Deep aquifer

D1 0.0192 0.0581 0.000 10.1 4,067 NP 1954.5 1947.5 1953 CFC-11,12,113 Late 1940s to early 1950s

D1 0.0152 0.0782 0.000 10.1 4,067 1955.5 1947 1953

D1 0.0192 0.0622 0.000 10.1 4,067 1954.5 1947.5 1953

D2 0.0372 0.435 0.007 9.65 3,983 2–7 1964 1950 1959 CFC-12,113 Early 1950s

D2 0.0392 0.169 0.006 9.65 3,983 1959 1950.5 1958

D2 0.0414 0.531 0.006 9.65 3,983 1965.5 1950.5 1958

D3 0.765 1.06 0.102 14.7 3,929 29–55 1971.5 1974 1980 CFC-11,12 Early to mid 1970s

D3 0.791 1.08 0.116 14.7 3,929 1971.5 1974 1981

D3 0.776 1.05 0.100 16.7 3,929 1972 1975 1981

D4 ERR 0.809 0.082 18.2 3,870 26–56 1971 ERR 1980 CFC-11,12 Early to mid 1970s

D4 0.630 0.808 0.073 18.2 3,870 1971 1973.5 1979

D4 0.622 0.784 0.079 18.2 3,870 1970.5 1973.5 1979.5

D5 0.135 0.198 0.045 19.0 3,862 46–69 1962.5 1961.5 1975.5 Early 1960s

D5 0.138 0.192 0.042 19.0 3,862 1962.5 1961.5 1975.5

D5 0.131 0.185 0.042 19.0 3,862 1962 1961 1975.5

D6 0.374 0.388 0.053 14.1 3,649 22–23 1965 1967.5 1975 CFC,11,12 Mid 1960s

D6 0.338 0.355 0.042 14.1 3,649 1964.5 1966.5 1973

D6 0.333 0.346 0.039 14.1 3,649 19–39 1964 1966.5 1972.5

D7 0.450 0.551 0.053 18.7 3,855 1968.5 1970.5 1977 CFC-11,12 Late 1960s to early 1970s

D7 0.448 0.548 0.054 18.7 3,855 1968.5 1970.5 1977
Concentration and Distribution of Geochemical Tracers 25

tac13-0813_fig07

0 1,000 2,000 3,000 METERS

0 3,000 6,000 9,000 FEET

3,500

3,000

2,500

4,000

Granite

Danskin

Mountains

A A’

Perched

Aquifer

Deep Aquifer

Sediments

FEET

ABOVE

NAVD 88

BEND INSECTION

BEND INSECTION

BEND INSECTION

BEND INSECTION

BEND INSECTION

BEND INSECTION

BEND INSECTION

2-7

29-55

92-94

26-56

1-6

NP

20-52

M1

PICR1

P2

P5 P3 D2

D4

D3 P1

D1

Upwelling of

geothermal water

EXPLANATION

Perched aquifer near 

Indian Creek Reservoir

Mountain system well

Perched aquifer well

Deep aquifer well

Flow path

Percentage of young 

water in sample

Not possible to 

determine

2-7

NP

VERTICAL EXAGGERATION ×20

60-78

34-57

76-77

Figure 7. Selected wells and percentages of young water in samples collected in the Mayfield area, 

southwestern Idaho, 2011–12.
26 Recharge Sources and Residence Times of Groundwater in the Mayfield Area, Southwestern Idaho, 2011–12

Table 10. Saturation indices of selected water samples from wells in the Mayfield area, southwestern Idaho, 2011–12. 

[Positive saturation indices indicate the solution is oversaturated with respect to that mineral. Negative saturation indices indicate the solution is undersaturated 

with respect to that mineral. Saturation indices based on log K (equilibrium constant) values being set to zero. Only selected saturation indices are shown here. 

Saturation indices calculated from the WATEQ-4 database]

Phase Formula Unique station identifer

M1 P1 P2 P3 P4 P5 D1

Albite NaAlSi3O8 -1.89 -0.43 -1.57 -1.43 -1.89 -1.42 -1.57

Anorthite CaAl2Si2O8 -5.29 -2.72 -5.40 -4.84 -5.46 -4.77 -4.73

Biotite K2Mg4Fe2Si6Al2O20F(OH)3 -262.92 -258.19 -259.24 -258.13 -262.87 -258.92 -255.31

Calcite CaCO3 -1.77 -1.66 -1.51 -1.40 -1.56 -1.38 -1.28

CO2(g) CO2 -1.83 -1.85 -1.69 -1.92 -1.71 -2.03 -1.98

Fluorite CaF2 -3.12 -2.61 -2.38 -2.67 -2.67 -2.40 -2.38

K-spar KAlSi3O8 -55.27 -53.93 -55.25 -55.05 -55.44 -54.82 -54.62

Montmorillonite-Ca Ca0.165Al2.33Si3.67O10(OH)2 3.10 6.03 3.01 3.19 2.72 3.08 2.84

Plag_(An50) CaNaAl3Si5O16 -146.95 -143.03 -146.80 -146.00 -147.03 -145.72 -145.32

Quartz SiO2 1.00 1.02 1.10 1.03 0.96 1.05 1.04

SiO2(a) SiO2 -0.34 -0.33 -0.25 -0.31 -0.38 -0.29 -0.27

Phase Formula Unique station identifer

D2 D3 D4 D5 D6 D7 SW1

Albite NaAlSi3O8 -0.53 -1.73 -2.15 -0.98 -1.11 -1.51 0.61

Anorthite CaAl2Si2O8 -2.62 -4.72 -5.13 -3.27 -4.17 -4.14 -1.29

Biotite K2Mg4Fe2Si6Al2O20F(OH)3 -252.75 -253.88 -246.39 -249.34 -244.25 -250.24 -265.90

Calcite CaCO3 -1.38 -0.76 0.02 -0.88 -0.27 -0.50 -2.47

CO2(g) CO2 -2.00 -2.46 -3.01 -2.22 -2.95 -2.39 -2.00

Fluorite CaF2 -2.80 -2.83 -2.31 -2.36 -2.19 -5.51 -4.74

K-spar KAlSi3O8 -53.56 -54.77 -55.01 -54.03 -53.32 -54.23 -54.18

Montmorillonite-Ca Ca0.165Al2.33Si3.67O10(OH)2 4.97 1.63 -0.88 3.43 0.76 1.69 9.66

Plag_(An50) CaNaAl3Si5O16 -141.58 -145.24 -145.14 -142.69 -142.84 -143.64 -143.58

Quartz SiO2 0.93 0.95 0.75 0.89 0.98 0.91 1.07

SiO2(a) SiO2 -0.36 -0.35 -0.53 -0.41 -0.28 -0.37 -0.37
Concentration and Distribution of Geochemical Tracers 27

Table 11. Geochemical model simulations results using NETPATH, measured carbon-14 and calculated carbon-14 ages, and flow path 

modeling travel times for groundwater in the Mayfield area, southwestern Idaho, 2011–12.

[Wells listed by unique well identifier. Positive values indicate dissolution. Negative values indicate precipitation. Dilution and evaporation were not allowed. 

Modeled carbon-13 values matched observed carbon-13 values. Age calculated by NETPATH is the travel time between two modeled wells. “Modern” indicates 

water recharged since the 1950s. Plag-an45: plagioclase with 45 percent anorthite composition. Abbreviations: CO2, carbon dioxide; pmc, percent modern 

carbon; SiO2, silicon dioxide]

Initial 

well

Final 

well SiO2 Calcite Ferric hydroxide

Potassium 

feldspar Biotite Camontmorillinite

CO2

gas Pyrite Plag-an45 Fluorite

Measured 

carbon-14

(pmc) 

final well

Age at final 

well from 

carbon -13 

mixing model

(years)

Calculated 

travel time 

NETPATH 

(years)

Mountain system to perched aquifer

M1 P2 -9.26190 0.36512 -0.05005 6.50415 0.02825 -3.08915 0.54810 0.00817 0.45884 0.00421 90.4 1,400 Modern

M1 P4 -8.93841 0.36130 -0.05760 6.10758 0.02414 -2.96640 0.26300 0.02140 0.53793 0.00184 111.3 Modern Modern

Perched aquifer to perched aquifer

P3 P4 -1.90745 0.11104 -0.02327 1.23607 0.00192 -0.59040 0.26919 0.01999 0.09493 0.00027 111.3 Modern Modern

Deep aquifer to deep aquifer

D3 D5 -5.89036 0.12706 -0.08175 3.93132 0.03977 -2.09806 0.04401 0.02296 0.63284 0.00343 55.3 6,700 1,222

D3 D7 -4.07992 0.15373 -0.05053 2.78184 0.02414 -1.46518 0.21932 0.01432 0.41929 0.00211 54.7 6,800 1,578

Only five of the modeled flow paths produced model 

simulation results that matched the observed chemical 

concentrations in the water samples collected from the study 

(table 11); two of these five flow paths are shown in figure 7. 

Of the five flow paths, two flow paths produced successful 

models but need to be interpreted with caution; the two models 

generated in the deep aquifer (from D3 to D5 and D3 to D7) 

are not along realistic groundwater flow paths. Water from 

well D3 was used as a representative upgradient water source 

to these wells but is not along a realistic flow path. Calculated 

travel times between wells were estimated from carbon-14 

and reactions with mineralogy (table 11). The corrected travel 

time, or the time it took for the water to travel between wells, 

ranged from 0 (modern) to 1,578 years. It should be noted that 

the travel time does not necessarily represent the age of the 

final water. 

Mixing models provide an understanding of the possible 

sources of recharge to groundwater in the study area (table 12) 

by mixing a potential source of recharge into an existing 

flow path. Recharge water was represented by surface water 

(from the Indian Creek sample), or geothermal water (from 

Sacajawea Hot Spring or from Boise Geothermal water). 

Water from the perched aquifer also was allowed to act as an 

input water to mimic vertical leakage between the perched and 

deep aquifer. The Indian Creek sample seemed to be a valid 

input to all simulated flow paths from the mountain system to 

the perched aquifer and between wells in the perched aquifer; 

surface water contributed between 20 and 99 percent of water 

to the perched aquifer in these models. One simulated flow 

path successfully modeled Indian Creek mixing into a flow 

path within the deep aquifer. However, there were not any 

valid models that mixed Indian Creek along a flow path from 

the mountain system to the deep aquifer. It is important to note 

that mixing models with Indian Creek were not constrained 

by carbon-13 values, so these models are presented with 

less confidence than other mixing models. Mixing models 

incorporating geothermal water allowed for the precipitation 

of calcite and fluorite due to changing temperature and 

saturation levels associated with cooling. Geothermal water 

mixed into flow paths from the mountain system to the deep 

aquifer did not produce any successful models. However, 

flow paths between wells within the deep aquifer were 

successfully modeled by mixing geothermal water; the final 

well appeared to receive between 1 and 23 percent of recharge 

from a geothermal source. Water from the perched aquifer did 

not successfully mix into any flow paths in the deep aquifer. 

Mixing models using surface water or geothermal water did 

not always produce valid models, suggesting that neither 

source solely accounts for recharge to groundwater. 
28 Recharge Sources and Residence Times of Groundwater in the Mayfield Area, Southwestern Idaho, 2011–12

Table 12. Validity of mixing models using NETPATH, Mayfield area, southwestern Idaho, 2011–12.

[Wells listed by unique well identifier. Mixing success denotes ability of flow path to be modeled with input water. Abbreviaton: NA, not applicable]

Initial

well Input water Final well Mixing success

Contribution from 

initial well

(percent)

Contribution from

input water

(percent)

M1 Indian Creek near Mayfield P2 Successful 38–40 60–62

M1 Indian Creek near Mayfield P3 Successful 36 64

M1 Indian Creek near Mayfield P4 Successful 4–35 65–96

M1 Indian Creek near Mayfield P5 Successful 1–40 60–99

M1 Indian Creek near Mayfield P1 Successful 50–75 25–50

P2 Indian Creek near Mayfield P3 Successful 40–43 57–60

P2 Indian Creek near Mayfield P4 Successful 50–63 37–50

P2 Indian Creek near Mayfield P5 Successful 16–20 80–84

P2 Indian Creek near Mayfield D3 Successful 1 99

P3 Indian Creek near Mayfield P4 Successful 80 20

P3 Indian Creek near Mayfield P5 Successful 40–47 53–60

P3 Indian Creek near Mayfield P1 Successful 28–36 64–72

P4 Indian Creek near Mayfield P5 Successful 22–27 73–78

P4 Indian Creek near Mayfield P1 Successful 10–21 79–90

P5 Indian Creek near Mayfield P1 Successful 57–77 23–43

D1 Indian Creek near Mayfield D2 Not Successful NA NA

D1 Indian Creek near Mayfield D3 Not Successful NA NA

D1 Indian Creek near Mayfield D4 Not Successful NA NA

D1 Indian Creek near Mayfield D6 Not Successful NA NA

D2 Indian Creek near Mayfield D3 Successful 28 72

D2 Indian Creek near Mayfield D4 Not Successful NA NA

D2 Indian Creek near Mayfield D6 Not Successful NA NA

D3 Indian Creek near Mayfield D4 Not Successful NA NA

D3 Indian Creek near Mayfield D6 Not Successful NA NA

D5 Indian Creek near Mayfield D7 Not Successful NA NA

M1 Indian Creek near Mayfield D1 Not Successful NA NA

M1 Indian Creek near Mayfield D2 Not Successful NA NA

M1 Indian Creek near Mayfield D3 Not Successful NA NA

M1 Indian Creek near Mayfield D4 Not Successful NA NA

M1 Indian Creek near Mayfield D5 Not Successful NA NA

M2 Indian Creek near Mayfield D6 Not Successful NA NA

M3 Indian Creek near Mayfield D7 Not Successful NA NA

M1 Sacajawea Hot Spring D1 Not Successful NA NA

M1 Sacajawea Hot Spring D2 Not Successful NA NA

M1 Sacajawea Hot Spring D3 Not Successful NA NA

M1 Sacajawea Hot Spring D4 Not Successful NA NA

M1 Sacajawea Hot Spring D5 Not Successful NA NA

M1 Sacajawea Hot Spring D6 Not Successful NA NA

M1 Sacajawea Hot Spring D7 Not Successful NA NA

M1 Boise Geothermal D1 Not Successful NA NA

M1 Boise Geothermal D2 Not Successful NA NA

M1 Boise Geothermal D3 Not Successful NA NA

M1 Boise Geothermal D4 Not Successful NA NA

M1 Boise Geothermal D5 Not Successful NA NA

M1 Boise Geothermal D6 Not Successful NA NA

M1 Boise Geothermal D7 Not Successful NA NA
Concentration and Distribution of Geochemical Tracers 29

Initial

well Input water Final well Mixing success

Contribution from 

initial well

(percent)

Contribution from

input water

(percent)

D1 Sacajawea Hot Spring D2 Not Successful NA NA

D1 Sacajawea Hot Spring D3 Not Successful NA NA

D1 Sacajawea Hot Spring D4 Not Successful NA NA

D1 Sacajawea Hot Spring D6 Not Successful NA NA

D1 Boise Geothermal D2 Not Successful NA NA

D1 Boise Geothermal D3 Not Successful NA NA

D1 Boise Geothermal D4 Not Successful NA NA

D1 Boise Geothermal D6 Not Successful NA NA

D2 Sacajawea Hot Spring D3 Not Successful NA NA

D2 Sacajawea Hot Spring D4 Successful 90–99 1–10

D2 Sacajawea Hot Spring D6 Successful 98 2

D2 Boise Geothermal D3 Not Successful NA NA

D2 Boise Geothermal D4 Successful 86–99 1–14

D2 Boise Geothermal D6 Successful 98 2

D3 Sacajawea Hot Spring D4 Successful 92–99 1–8

D3 Sacajawea Hot Spring D6 Successful 80–81 19–20

D5 Sacajawea Hot Spring D7 Not Successful NA NA

D3 Boise Geothermal D4 Successful 88–99 1–12

D3 Boise Geothermal D6 Successful 77 23

D5 Boise Geothermal D7 Not Successful NA NA

D1 P3 D2 Not Successful NA NA

D1 P3 D3 Not Successful NA NA

D1 P3 D4 Not Successful NA NA

D1 P3 D6 Not Successful NA NA

D1 P4 D2 Not Successful NA NA

D1 P4 D3 Not Successful NA NA

D1 P4 D4 Not Successful NA NA

D1 P4 D6 Not Successful NA NA

D2 P3 D3 Not Successful NA NA

D2 P3 D4 Not Successful NA NA

D2 P3 D6 Not Successful NA NA

D2 P4 D3 Not Successful NA NA

D2 P4 D4 Not Successful NA NA

D2 P4 D6 Not Successful NA NA

D3 P1 D4 Not Successful NA NA

D3 P1 D6 Not Successful NA NA

D4 P1 D6 Not Successful NA NA

D5 P1 D7 Not Successful NA NA

D3 P5 D4 Not Successful NA NA

D3 P5 D6 Not Successful NA NA

D4 P5 D6 Not Successful NA NA

D5 P5 D7 Not Successful NA NA

M1 P5 D1 Not Successful NA NA

M1 P1 D1 Not Successful NA NA

Table 12. Validity of mixing models using NETPATH, Mayfield area, southwestern Idaho, 2011–12.—Continued

[Wells listed by unique well identifier. Mixing success denotes ability of flow path to be modeled with input water. Abbreviation: NA, not applicable]
30 Recharge Sources and Residence Times of Groundwater in the Mayfield Area, Southwestern Idaho, 2011–12

Geochemical Evidence for Recharge from 

Upwelling Geothermal Waters 

The chemistry of the water in some of the deep wells 

in the Mayfield area has similar characteristics to other 

geothermally influenced water in Idaho; previous reports 

(Welhan, 2012) hypothesized that geothermal water may 

contribute recharge to the area. Wood and Low’s (1988) 

conceptual model of the western SRP suggests interaction 

between a deeper, geothermal aquifer consisting of 

interbedded rhyolite and volcanic sediments and an upper, 

cold-water aquifer composed of sediments and basalts of the 

Idaho and Snake River Groups. This model suggested that 

geothermal recharge in the western SRP is primarily sourced 

from precipitation falling on the flanks of the basin, which 

moves into the geothermal system, becomes heated, and 

slowly discharges to the cold-water aquifer through fractures. 

Welhan (2012) suggested that the same geothermal source that 

supplies the Boise Geothermal system and the hot springs of 

the Idaho Batholith may contribute significantly to the deep 

aquifer in the Mayfield area; chemistry can help to determine 

if a geothermal signature is present in groundwater in the deep 

aquifer. Wood and Low (1988) identified geothermal waters 

as having higher concentrations of sodium, bicarbonate, silica, 

fluoride, sulfate, chloride, arsenic, boron, and lithium and 

lower concentrations of calcium and magnesium than those in 

the cold-water aquifer. Water from some of the sampled wells 

in the deep aquifer in the Mayfield area exhibits some, but 

not all, of these chemical characteristics as compared to the 

perched aquifer; samples from the deep aquifer generally had 

higher concentrations of bicarbonate, arsenic, and lithium than 

those in the perched aquifer and they had lower concentrations 

of magnesium (figs. 3 and 5). Water type, as discussed above, 

in one deep aquifer samples (D6) is similar to that of Boise 

geothermal groundwater (sodium-bicarbonate type water) 

(Waag and Wood, 1987), suggesting a similarities between this 

well and geothermal groundwater.

The deep aquifer did not exhibit the high fluoride 

concentrations typical of Boise Geothermal water (deep 

aquifer, 0.19–0.51 mg/L; Boise Geothermal water, 8 to 

26 mg/L) (Waag and Wood, 1987). Welhan hypothesized that 

fluorite solubility might control the fluoride concentrations 

in geothermal water and that extremely high fluoride 

concentrations in deep aquifer samples are not likely. 

Similarly, the deep aquifer water did not exhibit the high 

chloride concentrations typical of Idaho Batholith waters (deep 

aquifer, 2.3–7.6 mg/L; Boise geothermal water, 10–50 mg/L) 

(Waag and Wood, 1987), suggesting that an Idaho Batholith 

geothermal water is not the predominant source of water to 

the comparatively dilute water found in the deep aquifer in the 

Mayfield area. 

Water temperature had a positive correlation with depth, 

suggesting a geothermal component to water in the deep 

aquifer, but Welhan (2012) showed that water temperatures 

in the study area are only slightly higher than would be 

explained by the geothermal gradient for the region. A plot of 

pH and fluoride concentrations in samples (fig. 8) is capable 

of distinguishing water derived from a cold-water source 

from water influenced by a geothermal source (suggested by 

Waag and Wood, 1987). Samples from wells D4 and D6 plot 

separately from the perched aquifer samples in a plot of pH 

and fluoride, suggesting that the sources of water to these two 

wells is unique and that these two wells may be influenced by 

geothermal water (fig. 8). 

A ternary diagram (fig. 9) with exaggerated lithium 

and boron concentrations was constructed to plot thermal 

waters from the Idaho Batholith (Berkeley Group, Inc., 1990; 

Parliman and Young, 1992; Druschel and Rosenberg, 2000) 

and water samples from the study area. Five samples collected 

from the deep aquifer plot close to thermal waters from the 

Idaho Batholith, suggesting geothermal water similar to that of 

the Idaho Batholith may recharge the deep aquifer. Only one 

sample from the perched aquifer plots near the Idaho Batholith 

thermal waters; this sample is the most upgradient sample in 

the perched aquifer and the closest to the mountain system. 

The surface-water sample plots near this signature, indicating 

that this sample was influenced by Idaho Batholith geology. 

All other perched aquifer samples plot far from the thermal 

water signature, suggesting these samples are not influenced 

by Idaho Batholith geology. Samples from wells D5 and D6 

plot between the perched aquifer samples and the signature 

of the Idaho Batholith, indicating these samples carry the 

signature of both the perched and deep aquifers.

Welhan (2012) suggested that geothermal recharge in 

the Mayfield area originates from the same source as the 

Boise geothermal system, as evidenced by elevated fluoride, 

lithium, and boron concentrations. Geochemical mixing 

models confirmed that geothermal water may be added along 

a flow path between wells within the deep aquifer, suggesting 

that geothermal water may be recharging the aquifer in 

proportions consistent with those hypothesized by Welhan 

(2012). A variation in chemistry evident between two points 

in the deep aquifer (D3 and D4) suggests that geothermal 

water is introduced between points D3 and D4 (figs. 3, 5, 

and 7; table 2). Mixing models confirm that water from well 

D4 could contain geothermal water, implying that upwelling 

geothermal water may enter the study area to the northeast of 

Highway 84. 
Concentration and Distribution of Geochemical Tracers 31

Figure 8. pH and fluoride concentrations in water samples collected from wells in Mayfield 

area, southwestern Idaho, 2011–12.

Figure 9. Water samples plotted with Idaho Batholith thermal water, Mayfield area, southwestern Idaho, 2011–12. For 

plotting purposes, boron concentrations were multiplied by 25 and lithium concentrations were multiplied by 100. 

tac13-0813_fig08

0 

0.1 

0.2 

0.3 

0.4 

0.5 

0.6 

6 6.5 7 7.5 8 8.5 

Fluoride, in milligrams per liter 

pH 

Perched aquifer near 

Indian Creek Reservoir 

and unique identifier 

Mountain system and 

unique identifier 

Perched aquifer and 

unique identifier 

Deep aquifer and 

unique identifier 

EXPLANATION

M1

P2

P3

P5

P4

P1

D3

D6

D7

D2

D1

D4

D5

PICR1

M1

P5

D6

PICR1

Percent lithium

Percent boron

Percent chloride

Seawater

Granite

Basalt

Limestone

Shale Sandstone Ultramafic

Diorite

EXPLANATION

Perched aquifer near

Indian Creek Reservoir 

and unique identifier

Mountain system and 

unique identifier

Perched aquifer and 

unique identifier

Deep aquifer and 

unique identifier

Surface water and 

unique identifier 

Rock assemblage

Idaho batholith

thermal waters

 80

 60

 40

 20

 20

 40

 60

 80 20

 40

 60

 80

M1

SW1

P2

P3

P5

P4

P1

D3

D6

D7

D2

D1

D4

D5

PICR1

M1

P5

D6

SW1

PICR1
32 Recharge Sources and Residence Times of Groundwater in the Mayfield Area, Southwestern Idaho, 2011–12

Sources of Recharge as Suggested by 

Geochemical Tracers

Concepts described in the Geochemical Evidence for 

Recharge from Upwelling Geothermal Waters section of this 

report suggest that various sources recharge groundwater in 

the study area. Identification of these sources of recharge can 

help to improve the current conceptual models of the study 

area and guide future decision making.

Oxygen-18 and deuterium signatures suggest that 

various sources of water exist in the study area (fig. 6). 

Water samples collected from the perched aquifer were more 

enriched (plot more positively) than samples from the deep 

aquifer; water from the mountain system had ratios that were 

similar to the perched aquifer, indicating a similar source of 

recharge. The more enriched signature seen in the perched 

aquifer could suggest that recharge occurred either at a lower 

altitude or during warmer precipitation events. The general 

climate and the hydrograph of Indian Creek (fig. 2) suggest 

that precipitation-sourced recharge is negligible during the 

summer because very little precipitation falls during warmer 

months. Therefore, the perched aquifer’s isotopic signature 

suggests that recharge likely occurs as winter precipitation 

at low elevations (most likely on the alluvial fan). The more 

depleted δ18O values in the deep aquifer suggest a dominant 

source of recharge from a higher altitude such as the Danskin 

Mountains, or from recharge during colder months. The deep 

aquifer could also receive recharge from numerous sources 

and have an average signature of depleted δ18O values. 

Groundwater samples plot more positively than the winter 

runoff collected from Indian Creek, suggesting that either 

groundwater recharge occurs at a lower altitude than the 

upper Indian Creek drainage basin (represented by the surface 

water sample) or that precipitation is sourced from warmer 

weather events such as spring precipitation. All groundwater 

samples plot as more enriched than the mean geothermal water 

signature, but are still within a feasible range of geothermal 

values, suggesting that an upwelling geothermal water source 

could also influence stable isotopic values.

The percentage of young water in each sample 

(calculated by ratio dating with CFCs) plotted in cross section 

helps to confirm concepts of recharge (fig. 7). The mountain 

system well (M1) contains 20–52 percent young water, 

suggesting that fractures in the mountain system facilitate 

delivery of mountain precipitation to depth. The percentage 

of young water in P2, the next well downgradient, is much 

lower (between 1 and 6 percent), suggesting that although 

some modern water may recharge this well, a local confining 

unit may prevent recharge from precipitation and (or) young 

water from the mountain system. Samples downgradient 

show a larger percentage of young water, indicating that the 

perched aquifer receives modern recharge from precipitation 

and (or) surface-water infiltration. The deep aquifer shows 

an increasing percentage of young water downgradient, 

suggesting that young water may be recharging the deep 

aquifer by natural percolation, or from pumping activity that 

may induce mixing with younger, shallower water. Mixing 

models did not successfully model perched aquifer water 

mixing into flow paths between deep aquifer wells, suggesting 

that percolating precipitation or surface water provides 

modern recharge to the deep aquifer. This conclusion seems 

valid when paired with nutrient data; water samples from the 

deep aquifer contained CFCs but not high levels of nutrients, 

suggesting that the perched aquifer may receive more recharge 

from local application of high-nutrient water, whereas the deep 

aquifer receives low-nutrient recharge from a larger regional 

area.

Deviation in physical parameter, major ion, trace 

element, and nutrient data along a flow path in the perched 

aquifer and deep aquifer can identify areas experiencing 

recharge. Specific conductivities in sites D1, D2, and D3 are 

lower than those measured in the perched aquifer or farther 

downgradient in the deep aquifer (table 3); these sites most 

likely receive a component of recharge from precipitation or 

from dilute surface water that infiltrate through fractures and 

faults in the area. A change in the geochemical composition of 

deep groundwater is apparent between two wells (D3 and D4) 

(figs. 3 and 5) in the deep aquifer. Between these two points, 

sodium, fluoride and sulfate concentrations increase, whereas 

magnesium and arsenic concentrations decrease. These 

changes are likely attributed to the area receiving recharge 

from a geothermal source that contributes sodium, sulfur, and 

fluoride. Another obvious point of inflection on major ion plots 

is around D6; this well is located at the terminus of the flow 

path and far from other wells in this flow path. Hydrologic and 

geologic controls affecting water quality in the deep aquifer 

seem to be different at well D6 than in any of the other deep 

wells, possibly due to its distance from other wells in the 

study area. 

Many lines of evidence indicate that there is an additional 

source of recharge for the deep aquifer than there is for the 

perched aquifer, such as an upwelling of geothermal water 

through fractures. The chemistry of the deep aquifer is 

somewhat similar to some geothermal water, suggesting that 

a component of the water in the deep aquifer is composed 

of geothermal water. Based on water temperatures, Welhan 

(2012) suggested that geothermal recharge to the Mayfield 

area may provide as much as 20 percent of the water to the 

deep aquifer composition. The simulation results of mixing 

models with geothermal end members suggest that geothermal 

water may comprise between 1 and 23 percent of the recharge 

to the deep aquifer. The composition of geothermal water 

that is actually present in the deep aquifer could be slightly 

different than the end members selected for the model; other 

processes, such as water-rock interaction, may be occurring 

in addition to mixing. The upwelling of older, warmer 

geothermal water explains the presence of older carbon-14 

ages and warm temperatures that are in the deep aquifer. 

Recharge from the mountain system also could be contributing 

to deep aquifer wells; this end member would most likely 

carry a signature similar to the Idaho Batholith and would 

show evidence of water-rock interaction.
Summary and Conclusions 33

The current conceptual model, taken from Welhan’s 

(2012) study, suggests that both cold and warm-water 

sources account for recharge to aquifers in the Mayfield area. 

Evidence from data collected during this study confirms this 

concept. Precipitation recharges the aquifer through direct 

application and through ephemeral surface-water recharge. 

Mixing of multiple sources is evidenced by nutrient data, 

geochemical modeling, and the overlap of oxygen-18 and 

deuterium signatures in water from the perched and deep 

aquifers. Cold‐water recharge moves vertically downward to 

the deep aquifer, bringing a modern source of recharge to the 

deep aquifer. The deep aquifer appears to receive recharge 

from the geothermal system as well, resulting in water with 

a geochemistry reflective of both modern, cold water and 

a geothermal component. Pumping may mix these various 

sources of water, either in the well or in the aquifer. Welhan 

(2012) also suggested that the proportions of recharge from 

these sources vary seasonally as a result of local pumping; 

local pumping could in fact induce surface-water recharge, 

mixing of water between aquifer components, or pull from a 

specific layer of an aquifer. 

Summary and Conclusions

Fifteen water samples were collected from a network of 

14 wells and 1 surface-water site in a relatively undeveloped 

area near Mayfield, Idaho. These samples were analyzed to 

determine the sources of recharge to the area aquifers, and 

the residence time of groundwater within distinct aquifers. 

Data and previously existing hydrologic information 

from the area were used in geochemical modeling and 

flow-path interpretations.

Distinct geochemical changes were observed along an 

assumed flow path, indicating areas of unique chemistry. 

A flow path in the perched aquifer showed variations in 

chemistry, suggesting that geochemical trends in the perched 

aquifer may be a product of depth or compartmentalization 

more than of distance in the flow path. Water in the deep 

aquifer may be diluted upgradient by recharge infiltration and 

influenced by another water source downgradient. A chemical 

variation was noted in concentrations between points D3 

and D4; based on observed differences in the geochemical 

composition of waters in some of the deep aquifer wells, the 

upwelling of geothermal water is indicated to be occurring in 

between these wells.

Geochemical differences were noted between the 

perched and deep aquifers, suggesting that these aquifers are 

composed of different sources of recharge or are mixtures of 

various sources. Higher nutrient concentrations were detected 

in the perched aquifer, which also contained water with 

younger carbon-14 dates. These chemical signatures in the 

perched aquifer suggest a modern, anthropogenic source of 

high‐nutrient water. Lower nutrient concentrations and a lower 

component of young water in the deep aquifer indicate that 

these two aquifers are somewhat distinct. Similarly, metals and 

trace element concentrations and oxygen-18 and deuterium 

signatures varied between the two aquifers, suggesting that 

these aquifers have different sources of water and different 

geochemical controls.

Recharge dates calculated by carbon-14 and 

chlorofluorocarbons (CFCs) varied. All groundwater samples 

contained detectable CFCs, indicating that they had at least a 

component of young recharge. Carbon-14 that was corrected 

using an open system carbon-13 mixing model indicated that 

water varied in age between 0 and 10,000 years. CFCs did 

not show a one-directional change along the deep-aquifer 

flow path. The presence of conflicting recharge dates suggests 

that recharge to the deep aquifer is a mixture of younger and 

older water.

Data from this study indicate modern recharge to aquifers 

in the Mayfield area is sourced from meteoric precipitation in 

and upgradient of the study area, infiltration of surface water 

from streams, and upwelling of geothermal water. Age tracers, 

elevated nutrient concentrations, and oxygen-18 signatures in 

water from the perched aquifer and mountain system suggest 

recharge to these systems is from winter precipitation and 

infiltration of ephemeral surface water. Young water in the 

deep aquifer is mostly likely recharged from high-elevation 

precipitation or surface water. However, it is apparent 

that another source of water having distinct geochemical 

characteristics is contributing recharge to the deep aquifer. The 

most likely source of this unique geochemical signature seen 

in the deep aquifer is the upwelling of geothermal water. 

Water temperature data, geochemical results, and mixing 

models suggest that the deep aquifer may receive recharge 

from a geothermal source. Elevated concentrations of trace 

elements typical of geothermal water were detected in water 

from the deep aquifer; the major change in chemistry that may 

suggest upwelling geothermal water is seen between wells D3 

and D4. Water in the deep aquifer generally carried a signature 

of geothermal water, suggesting at least a component of 

recharge to the deep aquifer comes from a geothermal source. 

Mixing models suggest that upwelling of geothermal water 

may contribute between 1 and 23 percent of recharge to the 

deep aquifer, similar to the proportion of geothermal water that 

Welhan (2012) hypothesized was recharging the deep aquifer.

Only five flow paths could be successfully modeled 

without mixing, indicating that the assumed flow path for this 

study may be incorrect or too simple. Variations in chemistry 

in the perched aquifer and differences in ages between 

samples suggest that the wells in the perched aquifer may not 

lie along a continuous flow path. At this time (2013), the role 

that faults and fractures play in facilitating groundwater flow 

is unknown. However, water samples collected from the most 

upgradient wells in the deep aquifer (D1, D2, and D3) suggest 

that dilute water is being transported to depth below the 

perched aquifer. The presence of CFCs in all samples indicates 

that some mechanism is allowing young water to enter the 

perched and deep aquifers and allowing recharge to occur.
34 Recharge Sources and Residence Times of Groundwater in the Mayfield Area, Southwestern Idaho, 2011–12

Acknowledgments 

Thanks to the well owners who gave permission to 

sample their wells and especially for their time and assistance. 

The following individuals provided valuable technical 

information, discussion and suggestions during the preparation 

of this report: Jim Bartolino and Gordon Rattray of the USGS, 

John Welhan of Idaho Geological Survey, Sean Vincent of 

IDWR, and Craig Tesch of IDWR. 

References Cited

Bendixsen, S., 1994, Summary of hydrologic conditions in the 

Mountain Home and Cinder Cone Butte areas: Boise, Idaho, 

Idaho Department of Water Resources, 17 p.

Berkeley Group, Inc., 1990, Boise geothermal aquifer study—

Final report: Boise, Idaho, Idaho Department of Water 

Resources, Contract No. DWR-89-03-46-102-54, 197 p.

Bureau of Reclamation, 2006, AgriMet—The Pacific 

Northwest cooperative agricultural weather network— 

AgriMet network map: Bureau of Reclamation, accessed 

Jan. 3, 2012, at http://www.usbr.gov/pn/agrimet/

agrimetmap/agrimap.html.

Church, J., 2007, Economic and population forecasts for 

Ada and Canyon Counties in Idaho (2007–2040): Boise, 

Idaho Economics, prepared for the Community Planning 

Association of Southwest Idaho (COMPASS), 87 p., 

accessed February 28, 2013, at http://www.compassidaho.

org/documents/prodserv/demo/JohnChurchForecast.pdf.

Clark, I.D., and Fritz, Peter, 1997, Environmental isotopes in 

hydrogeology: New York, Lewis Publishers, 328 p.

Coplen, T.B., 1993, Uses of environmental isotopes, in Alley, 

W.M., ed., Regional ground-water quality: New York, Van 

Nostrand Reinhold, p. 227–254. 

Coplen, T.B., 1996, New guidelines for reporting stable 

hydrogen, carbon, and oxygen isotope-ratio data: 

Geochemica et Cosmochimica Acta, v. 60, no. 17, 

p. 3,359–3,360. 

Coplen, T.B., Wildman, J.D., and Chen, Julie, 1991, 

Improvements in the gaseous hydrogen-water equilibration 

technique for hydrogen isotope ratio analysis: Analytical 

Chemistry, v. 63, p. 910–912.

Craig, Harmon, 1961, Isotopic variations in meteoric waters: 

Science, v. 133, no. 3,465, p. 1,702–1,703.

Druschel, K.D., and Rosenberg, P.E., 2000, Non-magmatic 

fracture-controlled hydrothermal systems in the Idaho 

Batholith—South Fork Payette geothermal system: 

Chemical Geology, v. 173, p. 271–291. 

Fishman, M.J., ed., 1993, Methods of analysis by the U.S. 

Geological Survey National Water Quality Laboratory—

Determination of inorganic and organic constituents in 

water and fluvial sediments: U.S. Geological Survey 

Open‐File Report 93-125, 217 p. 

Fishman, M.J., and Friedman, L.C., eds., 1989, Methods 

for determination of inorganic substances in water and 

fluvial sediments: U.S. Geological Survey Techniques of 

Water‐Resources Investigations, book 5, chap. A1, 545 p. 

(Also available at http://pubs.usgs.gov/twri/twri5-a1/.) 

Godfrey, B. ,2000, Köppen climate classification for the 

conterminous United States: Moscow, Idaho, University 

of Idaho, accessed November 14, 2012, at: http://inside.

uidaho.edu/appsOutput/metadataWAF/xml/Koppen

ClimateClassificationfortheConterminousUnitedStates.xml.

Hallberg, G.R., and Keeney, D.R., 1993, Nitrate, in Alley, 

W.M., ed., Regional ground-water quality: New York, Van 

Nostrand Reinhold, p. 297–322.

Hutchings, Jon, and Petrich, C.R., 2002, Ground water 

recharge and flow in the regional Treasure Valley aquifer 

system, geochemistry and isotope study: Boise, Idaho, 

Water Resources Research Institute, Research Report 

IWRRI-2002-08, 80 p.

Liberty, L.M., 2012, Geophysical characterization at the North 

Ada and East Ada sites–A 2012 Idaho Department of Water 

Resources report: Boise, Idaho, Boise State University, 

32 p.

Mayo, A.L., Muller, A.B., and Mitchell, J.C., 1984, 

Geochemical and isotopic investigations of thermal water 

occurrences of the Boise front area, Ada County, Idaho: 

Boise, Idaho, Idaho Department of Water Resources Water 

Information Bulletin 30, Part 14, 55 p.

Mitchell, J., 1981, Geological, hydrological, geochemical, 

and geophysical investigations of the Nampa-Caldwell 

and adjacent areas, southwestern Idaho: Boise, Idaho, 

Idaho Department of Water Resources Water Resources 

Information Bulletin 30, Part 11, 143 p.

National Agricultural Statistics Service, 2011, CropScapeCropland data layer, 2010: National Agricultural Statistics 

Service, accessed February 19, 2013, at http://nassgeodata.

gmu.edu/CropScape/. 

Natural Resources Conservation Service, 2010, Idaho Snow 

Survey Program: National Resources Conservation Service 

Web site, accessed February 20, 2013, at http://www.id.nrcs.

usda.gov/snow/.
References Cited 35

Neely, K.W., and Crockett, J.K., 1998, Ground water quality 

characterization and initial trend analyses for the Treasure 

Valley shallow and deep hydrogeologic subareas: Boise, 

Idaho, Idaho Department of Water Resources Water 

Information Bulletin 50, pt. 3, 76 p., 5 appendixes.

Newton, G.D., 1991, Geohydrology of the regional aquifer 

system, western Snake River Plain, southwestern Idaho: 

U.S. Geological Survey Professional Paper 1408-G, 

p. G1–G52, 1 pl.

Norton, M.A., Ondrechen, William, and Baggs, J.L., 1982, 

Groundwater investigation of the Mountain Home plateau, 

Idaho: Boise, Idaho, Idaho Department of Water Resources 

Open-File Report, 62 p.

O’Neil, J.M., and Lopez, D.A., 1985, Character and regional 

significance of Great Falls tectonic zone, east-central 

Idaho and west-central Montana: American Association of 

Petroleum Geologists Bulletin, v. 69, no. 3, p. 436–447.

Parliman, D.J., and Young, H.W., 1992, Compilation of 

selected data for thermal-water wells and springs in Idaho, 

1921 through 1991: U.S. Geological Survey Open-File 

Report 92-175, 201 p. (Also available at http://pubs.er.usgs.

gov/publication/ofr92175.) 

Pearson, F.J., Jr., and White, D.E., 1967, Carbon-14 ages and 

flow rates of water in the Carrizo Sand, Atascosa County, 

Texas: Water Resources Research, v. 3, no. 1, p. 251–261.

Peel, M.C., Finlayson, B.L., and McMahon, T.A., 2007, 

Updated world map of the Köppen-Geiger climate 

classification: Hydrology and Earth System Sciences, v. 11, 

p. 1,633–1,644, accessed February 20, 2013, at http://www.

hydrol-earth-syst-sci.net/11/1633/2007/hess-11-1633-2007.

pdf.

Phillips, W.M., Lewis, R.S., Gillerman, V.S., Garwood, D.L., 

and Stewart, D.E., 2012, Geologic map of the Mayfield 

area, Ada and Elmore Counties, Idaho: Idaho Geological 

Survey Digital Web Map DWM-144, scale 1:36,000.

Plummer, L.N., Michel, R.L., Thurman, E.M., and Glynn, 

P.D., 1993, Environmental tracers for age-dating young 

ground water, in Alley, W.M., ed., Regional ground-water 

quality: New York, Van Nostrand Reinhold, p. 255–294.

Plummer, L.N., Prestemon, E.C., and Parkhurst, D.L., 

1994, An interactive code (NETPATH) for modeling net 

geochemical reactions along a flow path version 2.0: 

U.S. Geological Survey Water-Resources Investigations 

Report 94-4169.

Révész, Kinga, and Coplen, T.B., 2008a, Determination of 

the δ(18O/16O) of water—RSIL lab code 489, chap. C2 

of Révész, Kinga, and Coplen, T.B., eds., Methods of the 

Reston Stable Isotope Laboratory: U.S. Geological Survey 

Techniques and Methods 10-C2, 28 p. (Also available at 

http://pubs.usgs.gov/tm/2007/tm10c2/.) 

Révész, Kinga, and Coplen, T.B., 2008b, Determination of 

the δ(2H/1H) of water—RSIL lab code 1574, chap. C1 of

Révész, Kinga, and Coplen, T.B., eds., Methods of the 

Reston Stable Isotope Laboratory: U.S. Geological Survey 

Techniques and Methods 10-C1, 27 p. (Also available at 

http://pubs.usgs.gov/tm/2007/tm10c1/.)

Schlegel, M.E., Mayo, A.L, Nelson, Steve, Tingey, Dave, 

Henderson, Rachel, and Eggett, Dennis, 2009, Paleoclimate of the Boise area, Idaho from the last glacial 

maximum to the present based on groundwater δ2H and 

δ18O compositions: Quaternary Research, v. 71, no. 2, 

p. 172–180.

Shervais, J.W., Shroff, G., Vetter, S.K., Matthews, S. Hanan, 

B.B., and McGee, J.J., 2002, Origin and evolution of the 

western Snake River Plain—Implications from stratigraphy, 

faulting, and the geochemistry of basalts near Mountain 

Home, Idaho: Idaho Geological Survey Bulletin 30, 

p. 343–361, accessed December 5, 2012, at http://www.usu.

edu/geo/shervais/Shervais_ElectronicPubList.htm.

SPF Water Engineering, 2007a, Ground-water supply 

evaluation for the Mayfield townsite property: SPF Water 

Engineering, 30 p., plus appendixes.

SPF Water Engineering, 2007b, Groundwater supply 

evaluation for Elk Creek Village, Application for Permit No. 

61-12090: SPF Water Engineering, 17 p., plus appendixes.

U.S. Department of Agriculture, 2012, Common land use 

GIS database: Salt Lake City, Utah, U.S. Department of 

Agriculture-FSA Aerial Photography Office, series number 

20051107.

U.S. Geological Survey, variously dated, National field manual 

for the collection of water-quality data: U.S. Geological 

Survey Techniques of Water-Resources Investigations, 

book 9, chaps. A1–A9. (Also available at http://pubs.water.

usgs.gov/twri9A.)

U.S. Geological Survey, 2006, The Reston Chlorofluorocarbon 

Laboratory—Dissolved gas N2/Ar and 4He sampling 

instructions: U.S. Geological Survey Web site, accessed 

February 20, 2013, at http://water.usgs.gov/lab/dissolvedgas/sampling/.

U.S. Geological Survey, 2009, The Reston Chlorofluorocarbon 

Laboratory—CFC bottle sampling method: U.S. Geological 

Survey Web site, accessed February 20, 2103, at http://

water.usgs.gov/lab/chlorofluorocarbons/sampling/bottles/.
36 Recharge Sources and Residence Times of Groundwater in the Mayfield Area, Southwestern Idaho, 2011–12

U.S. Geological Survey, 2011, National Water Information 

System (NWISWeb)—USGS water-quality data for Idaho: 

U.S. Geological Survey database, accessed February 20, 

2013, at http://waterdata.usgs.gov/id/nwis/qw.

U.S. Geological Survey, 2012a, The Reston 

Chlorofluorocarbon Laboratory—Analytical procedures 

for CFCs: U.S. Geological Survey Web site, accessed 

February 20, 2013, at http://water.usgs.gov/lab/

chlorofluorocarbons/lab/analytical_procedures/. 

U.S. Geological Survey, 2012b, The Reston 

Chlorofluorocarbon Laboratory—Analytical procedures for 

CFCs: U.S. Geological Survey Web site, accessed July 17, 

2012, at http://water.usgs.gov/lab/chlorofluorocarbons/lab/

analytical_procedures/.

Waag, C.J., and Wood, S.H., 1987, Evaluation of the Boise 

Geothermal System: Boise, Idaho, Boise State University, 

Department of Geology and Geophysics, Final report to 

Idaho Department of Water Resources, 168 p.

Welhan, J.A., 2012, Preliminary hydrogeologic analysis of 

the Mayfield area, Ada and Elmore Counties, Idaho: Idaho 

Geological Survey, 42 p.

Western Regional Climate Center, 2008, Idaho climate 

summaries: Western Regional Climate Center, accessed 

February 20, 2013, at http://www.wrcc.dri.edu/summary/

climsmid.html.

Whitehead, R.L., 1986, Geohydrologic framework of the 

Snake River Plain, Idaho and eastern Oregon: U.S. 

Geological Survey Hydrologic Investigations Atlas HA-681, 

3 sheets, scale 1:1,000,000.

Williams, M.L., and Etheridge, A.B., 2013, An evaluation 

of seepage gains and losses in Indian Creek Reservoir, 

Ada County, Idaho, April 2010–November 2011: 

U.S. Geological Survey Scientific Investigations 

Report 2013-5047, 26 p.

Wood, S.H., 1994, Seismic expression and geological 

significance of a lacustrine delta in Neogene deposits of the 

western Snake River Plain, Idaho: American Association of 

Petroleum Geologists Bulletin, v. 78, p. 102–121.

Wood, S.H., and Anderson, J.E., 1981, Geology, in Mitchell, 

J.C., ed., Geothermal investigations in Idaho, part 11, 

Geological, hydrological, geochemical, and geophysical 

investigations of the Nampa-Caldwell and adjacent areas, 

southwestern Idaho: Boise, Idaho, Idaho Department of 

Water Resources Water Information Bulletin 30, p. 9–31.

Wood, S.H., and Clemens, D.M., 2002, Geologic and tectonic 

history of the western Snake River Plain, Idaho and Oregon, 

in Bonnichsen, Bill, White, C.M., and McCurry, Michael, 

eds., Tectonic and magmatic evolution of the Snake River 

Plain volcanic province: Moscow, Idaho, Idaho Geological 

Survey Bulletin 30, p. 69–103.

Wood, W.W., and Low, W.H., 1988, Solute geochemistry of 

the Snake River Plain Regional Aquifer system, Idaho and 

eastern Oregon: U.S. Geological Survey Professional Paper, 

1408-D, 79 p.

Woods Hole Oceanographic Institution, 2003, Measuring 14C 

in seawater ΣCO2 by accelerator mass spectrometry: Woods 

Hole, Mass., WHP Operations and Methods, 9 p.
Publishing support provided by the U.S. Geological Survey 

Publishing Network, Tacoma Publishing Service Center

For more information concerning the research in this report, contact the 

Director, Idaho Water Science Center

U.S. Geological Survey 

230 Collins Road

Boise, Idaho 83702

http://id.water.usgs.gov
Hopkins— Recharge Sources and Residence Times of Groundwater, Mayfield Area, Southwestern Idaho, 2011–12—Scientific Investigations Report 2013–5115